src/uid_wrapper.c: check for uid_wrapper related syscall numbers before uid_wrapper_e...

src/uid_wrapper.c: check for uid_wrapper related syscall numbers before uid_wrapper_enabled()

On FreeBSD syscall() is called after the pthread_atfork() 'prepare'
hooks. So we need to avoid calling uid_wrapper_enabled() as it would
deadlock.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>