core: New flag ARGPARSE_FLAG_USERVERS to try versioned config files.

* src/gpg-error.h.in (ARGPARSE_FLAG_USERVERS): New.
* src/argparse.c: Include unistd.h.
(try_versioned_conffile): New.
(_gpgrt_argparser): Use it.

This is neded to bring the versioned config files back to gpg.

• Signed-off-by: Werner Koch <wk@gnupg.org>