systemctl: support auditd.service better

systemctl: support auditd.service better

We would print the filename header before trying to open the file. But since
the header was printed to stdout, and the error to stderr, the error would appear
on the terminal before the header. It is cleaner to open the file first, then
and only then print the header.

Also exit on first error. We shouldn't report success if we were unable to open
a file.