Fall back to SO_PEERCRED if credentials passing fails

Fall back to SO_PEERCRED if credentials passing fails

Turns out libdbus doesn't send struct ucred credentials on linux, but
just relies on the SO_PEERCRED support. However, gdbus does send, and
expect to recieve a ucred credential. So, when libdbus talks to a
gdbus server the authentication fails to send the credentials.

We fix this by falling back to g_socket_get_credentials() if we don't
get any credential messages.