Action: Detach from the running session

[whisperity]

Instead of having to call the monomux binary with a specific flag, the client itself should be able to send a request to detach, which would result in a graceful detachment (instead of a "kicked" message) of the client that sent the request.

Implementing this could also obsolete the -d/--detach flag. (Likely -D/--detach-all should be kept for scripting purposes.)