Various fixups r.e. revocation.

1) Don't enqueue messages to contacts who have revoked us.
2) Mark contacts who have revoked us in black.
3) Mark dead messages in black.
fix nogui build
No need for dev bool in noGUIClient.
Switch to late signing.

Previously, messages were signed for delivery (with the group signature)
as soon as they were enqueued. This resulted in a complex dance when
receiving a group update because messages in the outgoing queue needed
to be resigned.

This change switches to late signing: messages are only signed just
before transmission to the destination server.