s3: Allow child processes to exit gracefully if we are out of fds

s3: Allow child processes to exit gracefully if we are out of fds

When we run out of file descriptors for some reason, every new
connection forks a child that immediately panics causing smbd to
coredump.  This seems unnecessarily harsh; with this code change we
now catch that error and merely log a message about it and exit
without the core dump.

Signed-off-by: Tim Prouty <tprouty@samba.org>