NMActiveConnection:connection was an object path rather than an
NMRemoteConnection because in the past the NMObject property system
wasn't capable of handling NMRemoteConnection-valued properties.
NMActiveConnection:master was an object path rather than an NMDevice
entirely by accident. Fix both.

NMActiveConnection:specific-object can't currently be converted to an
object, because we don't know ahead of time what object type it is,
and NMObject can't deal with that. Instead, we rename it to
:specific-object-path (and likewise for its get function), both to
emphasize that it doesn't behave like other properties, and to leave
the old name open for an actual object-valued property later on.