shared: ensure errno always zero on entering main

standards do not require errno to be reset to zero before entering
main, third party libraries like openssl and libcap(1) use ELF
constructors that if buggy (1) will clobber errno value to everybody
else.

Libselinux also clobbers errno in most tests cases (2) ensure that is
set to zero before starting.

(1) https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=f25a1b7e69f7b33e6afb58b3e38f3450b7d2d9a0
(2) SELinuxProject/selinux#445

src/shared/main-func.h

@@ -20,6 +20,7 @@
 
 #define _DEFINE_MAIN_FUNCTION(intro, impl, result_to_exit_status, result_to_return_value) \
         int main(int argc, char *argv[]) {                              \
+                errno = 0;                                              \
                 int r;                                                  \
                 assert_se(argc > 0 && !isempty(argv[0]));               \
                 save_argc_argv(argc, argv);                             \

src/shared/tests.h

@@ -184,6 +184,7 @@ static inline int run_test_table(void) {
 
 #define DEFINE_TEST_MAIN_FULL(log_level, intro, outro)    \
         int main(int argc, char *argv[]) {                \
+                errno = 0;                                \
                 int (*_intro)(void) = intro;              \
                 int (*_outro)(void) = outro;              \
                 int _r, _q;                               \