server: Ensure log/auth directories exist at startup

The daemon startup had lots of swapping around of effective uid, and
aborted if the log/auth directories didn't have the expected permissions.

Honestly this makes no sense - we're uid 0, so let's just ensure
they're directories and call chown() ourself.  I have no idea what the
"paranoia" here is about - if someone had managed to e.g. make a
symbolic link in /var to somewhere unexpected, there are plenty of
other ways they could attack the system.

Rather than aborting, let's just call mkdir()/chown()/chmod() and
check the return values.

https://bugzilla.gnome.org/show_bug.cgi?id=684315