libmoonshot: use dbus_g_connection_open

libmoonshot: use dbus_g_connection_open

It turns out that opening a bus with dbus_connection_open and then
using dbus_connection_get_g_connection does not actually set up the
type serialization system so nothing works on the resulting bus.
Instead, use g_connections everywhere we can and explicitly get the
real connection for name owner and service start operations.