Use sysconf as a fallback mechanism in the initialization.
* src/posix-io.c (max_fds_fallback): New. [_SC_OPEN_MAX] (_gpgme_io_subsystem_init): Set max_fds_fallback. (get_max_fds): Use max_fds_fallback.
- GnuPG-bug-id: T7694
- Fixes-commit: e8e5434cc4f5cc3c322659792cc1850a5c5b26b8
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>