Fix (re-)detection of libmodbus RTU USB support with static libmodbus builds

[jimklimov]

...and allow users to require configure --with-modbus+usb to avoid surprises with a wrong choice of the library.

Should help with investigation for #2666 and fixes a bug detected in that discussion.

Also revise configuration script report.

Closes: #2676

Example runs with failsafe firing (where the OS has a packaged standard shared libmodbus; a custom static build with USB support is installed into --prefix=/tmp)

NEGATIVE CASES

• Use new options and cause use of OS-provided library (by default):

:: ./autogen.sh && ./configure --with-modbus+usb

• Legacy common case: explicitly request apc_modbus and libusb among configure options (and still use OS-provided library):

:; ./configure --with-drivers=apc_modbus --with-usb

• Legacy common case: explicitly request libmodbus and libusb, but passing --with-modbus-includes and/or --with-modbus-libs for a build that is not USB-capable (done here via bad location of --with-modbus-libs):

:; ./configure --with-modbus --with-usb --with-modbus-includes=-I/tmp/include/modbus --with-modbus-libs="-L/tmp/libxxx -lmodbus"

These all end with:

...
checking for modbus.h... yes
checking for modbus_new_rtu... yes
checking for modbus_new_tcp... yes
checking for modbus_set_byte_timeout... yes
checking for modbus_set_response_timeout... yes
checking for modbus_new_rtu_usb... no
configure: Retry detection of libmodbus USB support
checking for modbus_new_rtu_usb... no
configure: WARNING: Both --with-modbus and --with-usb were requested, and a libmodbus was found, but it seems to not support USB. You may require a custom build per https://github.com/networkupstools/nut/wiki/APC-UPS-with-Modbus-protocol
...
configure: error: modbus library variant with RTU USB support not found, required for USB-capable Modbus drivers

POSITIVE CASES

• Legacy common case: explicitly request libmodbus and libusb, explicitly passing correct --with-modbus-includes and --with-modbus-libs for a build that is USB-capable:

:; ./configure --with-modbus --with-usb --with-modbus-includes=-I/tmp/include/modbus --with-modbus-libs="-L/tmp/lib -lmodbus"

• Request the apc_modbus driver AND libusb, explicitly passing correct --with-modbus-includes and --with-modbus-libs for a build that is USB-capable:

:; ./configure --with-drivers=apc_modbus --with-usb --with-modbus-includes=-I/tmp/include/modbus --with-modbus-libs="-L/tmp/lib -lmodbus"

• Use new option and pass correct --with-modbus-includes and --with-modbus-libs for a build that is USB-capable:

:: ./configure --with-modbus+usb --with-modbus-includes=-I/tmp/include/modbus --with-modbus-libs="-L/tmp/lib -lmodbus"
```

These all end with:
```
...
checking for libmodbus version via pkg-config... 3.1.6 found
checking for libmodbus cflags... -I/tmp/include/modbus
checking for libmodbus ldflags... -L/tmp/lib -lmodbus
checking for modbus.h... yes
checking for modbus_new_rtu... yes
checking for modbus_new_tcp... yes
checking for modbus_set_byte_timeout... yes
checking for modbus_set_response_timeout... yes
checking for modbus_new_rtu_usb... no
configure: Retry detection of libmodbus USB support
checking for modbus_new_rtu_usb... yes
checking types of arguments for modbus_set_byte_timeout... sec_usec_uint32_cast_timeval_fields
Found types to use for modbus_set_byte_timeout: sec_usec_uint32_cast_timeval_fields
...
* build Modbus drivers: yes
* build Modbus drivers with RTU USB support:    yes

The resulting driver build reports its USB-capability early in debug log now (see second [D1] line) as well as via DRIVER_NAME seen in banner and protocol (upsc etc.):

:; ./drivers/apc_modbus -D -V
Network UPS Tools 2.8.2.1179.10-1189-geb99e125f (development iteration after 2.8.2) - NUT APC Modbus driver with USB support 0.10
   0.000000     [D1] Using USB implementation: libusb-1.0.25 (API: 0x1000109)
   0.000040     [D1] This build of the driver is USB-capable; also Serial and TCP Modbus RTU are supported
   0.000055     [D1] Network UPS Tools version 2.8.2.1179.10-1189-geb99e125f (development iteration after 2.8.2) built with gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0 and configured with flags: --with-modbus+usb --without-docs --with-modbus-includes=-I/tmp/include/modbus --with-modbus-libs='-L/tmp/lib -lmodbus'
   0.000088     [D1] upsnotify: notify about state 4 with libsystemd: was requested, but not running as a service unit now, will not spam more about it
   0.000117     [D1] upsnotify: failed to notify about state 4: no notification tech defined, will not spam more about it

NEUTRAL CASES

The apc_modbus driver is build using OS packaged libmodbus, without USB support, without aborting NUT build (common packaging).

• Explicitly requesting the apc_modbus driver but not explicitly requesting libusb:

:; ./configure --with-drivers=apc_modbus
...
* build Modbus drivers: yes
* build Modbus drivers with RTU USB support:    no
...

:; ./drivers/apc_modbus -D -V
Network UPS Tools 2.8.2.1179.10-1189-geb99e125f (development iteration after 2.8.2) - NUT APC Modbus driver without USB support 0.10
   0.000000     [D1] This build of the driver is not USB-capable, only Serial and TCP Modbus RTU are supported
   0.000030     [D1] Network UPS Tools version 2.8.2.1179.10-1189-geb99e125f (development iteration after 2.8.2) built with gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0 and configured with flags: --with-drivers=apc_modbus --without-docs
   0.000061     [D1] upsnotify: notify about state 4 with libsystemd: was requested, but not running as a service unit now, will not spam more about it
   0.000098     [D1] upsnotify: failed to notify about state 4: no notification tech defined, will not spam more about it

• Default build requesting everything (but not specific combos explicitly expected by fallbacks tested above):

:; ./configure --with-all

• Even a build with libusb and libmodbus but without custom options for the latter (nor the specific driver):

:; ./configure --without-docs --with-usb --with-modbus