dbus: Terminate cleanly on messagebus shutdown

dbus: Terminate cleanly on messagebus shutdown

By default, dbus_connection_dispatch() will call _exit() if the bus
connection has been closed. This caused wpa_supplicant to terminate
without properly cleaning up after itself.

To ensure that we terminate cleanly when the messagebus terminates,
override the exit_on_disconnect behavior and install a filter to handle
libdbus's "Disconnected" signal.

[Bug 474]

Signed-hostap: Daniel Gnoutcheff <daniel@gnoutcheff.name>