[krbdev.mit.edu #6618] Support optional creation of PID files for krb5kdc and kadmind

[Greg Hudson via RT]

This looks fine.  (I would have put the conditional on pid_file == NULL
into the helper, but it doesn't really matter.)  I looked over the
source to some other open-source daemons and it didn't look like
treating the pid file as a lock file is terribly common, so I think
simply writing out the pid unconditionally is fine.