bus: Implement GDBusAuthObserver callback
bus: Implement GDBusAuthObserver callback

ibus uses a GDBusServer with G_DBUS_SERVER_FLAGS_AUTHENTICATION_ALLOW_ANONYMOUS,
and doesn't set a GDBusAuthObserver, which allows anyone who can connect
to its AF_UNIX socket to authenticate and be authorized to send method calls.
It also seems to use an abstract AF_UNIX socket, which does not have
filesystem permissions, so the practical effect might be that a local
attacker can connect to another user's ibus service and make arbitrary
method calls.

BUGS=rhbz#1717958

ibus/ibus,fujiwarat/ibus,fujiwarat/ibus,ibus/ibus,ibus/ibus,ibus/ibus,fujiwarat/ibus,fujiwarat/ibus