printers: Added new async API to check availability of maintenance commands

This cleans the code up a bit so that we can extract part of the logic
from _pp_maintenance_command_execute_thread() before getting into fixing
the problem in the logic checking whether a CUPS command is available.

Besides, it will be useful to have this logic extracted as it will be used
later on from pp-printer-entry.c to know whether the "Clean" command is
available, in order to show a menu item "Clean Print Heads" (bug 764620).

https://bugzilla.gnome.org/show_bug.cgi?id=766861