RFCS: Add backdoors for using FDs with HSL IO

[fredemmott]

OS\FileDescriptor -> IO\Handle

If an OS\FileDescriptor is obtained/operated on via some unusual mechanism, there is currently no public way to convert it to an IO\Handle.

While we can't provide type safety - or even true runtime safety - as we're aiming for 100% completeness, we should provide a clearly-discouraged way to do this.

For example, perhaps a File\ReadWriteHandle could be created by File\fdopen_read_write_UNSAFE($fd); we could similarly add Unix\fdopen_UNSAFE() and TCP\fdopen_UNSAFE()

Accessing arbitrary file descriptors by number

This is currently intentionally disallowed due to:

• risk of confusion with STDOUT/STDERR/STDIN not being associated with the relevant _FILENO constants
• it allows breaking request encapsulation

For both of these:

• neither is an issue in standalone CLI mode
• neither is an issue in CLI server mode if the FD is taken from the CLI client, not the server
• the two issues are equivalent in HTTP or fastcgi server mode

It's also important to note that this isolation is really just making it harder, due to the presence of /dev/fd and/or /proc/self/fd (depending on the platform). On the one hand, as we don't really have the isolation we want it doesn't hurt as much - but, on the other hand, we don't need to add it as the OS already provides a mechanism