help_print_optstring: Don't depend on snprintf.

The current code calls snprintf unconditionally, even if NLS is disabled
(although the function is not called, the code is still compiled), so it
won't build in environments which lack this function.

For the NLS-enabled case we can rely on the GNU libintl fallback being
available as required.

Otherwise we can just provide a stub to ensure the dead code compiles,
and for the NLS test programs hack in sprintf as a replacement which
should work just fine.