It's a singleton, but PolicyKit didn't increment the reference count
when returning from polkit_authority_get() like we expected (which has
since been fixed upstream).  So for now, just don't unref the authority
at all.

Since we don't do that, there's a chance that some PolicyKit calls could
be outstanding when either the NMSysconfigSettings object or one of the
NMSysconfigConnection objects are around, so we make sure we cancel any
PolicyKit calls when the object gets disposed.  This is tricky, because
canceling them from the dispose may mean that the callback gets called
after the object is actually destroyed, so we have to be careful not to
access any private object data from the callbacks in that situation.