tui: fix unsetting Gateway (rh #1163896)

When the Gateway field is empty, we need to set the property to NULL,
not "".
cli: Finish waiting for device activation when they disconnect

When wifi secrets are missing, NM_DEVICE_STATE_FAILED due to
NM_DEVICE_STATE_REASON_NO_SECRETS is immediately followed by traversal to
NM_DEVICE_STATE_DISCONNECTED as soon as the server makes sure it won't
reconnect automatically. We sometimes aren't quick to handle the first signal
and only get the latter one.

Let's treat all states that aren't ordinarily reached upon activation as bad.
contrib: Require libselinux for RPM build

It ensures ifcfg-rh doesn't mess up the labels.
contrib: Enable PolicyKit agent in RPM