D-Bus: Add a dbus handler for expected disconnection

D-Bus: Add a dbus handler for expected disconnection

Add a global D-Bus handler ExpectDisconnect for setting
wpa_s->own_disconnect_req flag. This flag will prevent wpa_supplicant
from adding blacklists and requesting incomplete scan upon the incoming
disconnection. This is mainly meant for a case where suspend/resume is
used and some external component knows about that and can provide the
information to wpa_supplicant before the disconnection happens.

Signed-off-by: Ningyuan Wang <nywang@google.com>