dbus: Add SignalPoll() method to report current signal properties

Analogous to the control interface's SIGNAL_POLL request.

Signed-hostap: Dan Williams <dcbw@redhat.com>
It will be checked on below if the driver was able to successfully provide the info (though.. it would be nice to fill in the structure but it seems that it's done like in the rest of the wpa_supplicant anyways and this a cherry-picked upstream patch)

ret = wpa_drv_signal_poll(wpa_s, &si);
	if (ret) {