cfg: Fix implementation of timeout in attempting to lock the config file.

cfg: Fix implementation of timeout in attempting to lock the config file.

Without removing SA_RESTART from the SIGALRM handler, the fcntl call will
never return, even after the signal handler is invoked and returns.

We haven't seen a problem in practice, at least not recently, but that's
probably just luck combined with not holding the configuration file lock
for very long.