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>
[wpa_supplicant] Always return channel width with signal_poll
Merge pull request #17 from tigeli/signalpoll

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