In the dbus_connect() code path, return the subscription token.
In the Modem class, use this token to disconnect signal callbacks when an
interface is removed.
Generalize the signal connect/disconnect handling: have one tuple defining all
signals and their handler functions, add generalized loop to attach them. Store
all subscription tokens in a dict of lists, any number of signal callbacks per
general interface name. When an interface is announced to be removed from
ofono, detach all signals for that interface implicitly.
So far this only handles the MessageManager interface's IncomingMessage signal,
but others will likely follow soon.
Fixes: OS#2242
Change-Id: I0939ef414bc599ee8742df48da04d8d9569d00ba
47de6b0a24