Make printing default log size limit consistent

The default log size limit is 15M for most plug-ins. The printing
plug-in uses 50M. Set it to 15M to avoid very large reports.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Rename printing plug-in and add check for cups package

The 'printing' plug-in is almost entirely cups-specific (the
lpstat invocations would work with another lp implementation but
that's fairly unlikely..). Add a check for the package and rename
the module to cups.py.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Enable ppp plug-in on more distributions

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>