guest mode: invite users without giving them full access, give them a specific session only

[Antoine Martin]

Add an option to show other neighbouring users on the same server in "send to", as a separate sub-menu or action on the control window.

• server listens for mdns.winswitch-clients (re-use reverse MDNS code?)
• neighbouring winswitch clients broadcast (if they so choose) their ID and nickname (and maybe more?). The server (and the clients!) will have to time them out.
• send the list of known neighbours to all clients (add_unauthed_user)
• if a client wants to send a session to another client, it sends chosen other client ID (+ public key if known)
• server broadcasts (new mdns type? re-use?) the list of waiting guest sessions: sessionID:userID
• other client detects that there's a session waiting for its ID, connects to the server, and does a guest_login
• if other client's public key is not known (and the current client has not supplied it):
  • send current client a "verify_identify" message with the IP, public key, ID, session
  • current client ACKs (or NACKs) with "verify_identify_response"
• server sends other client the requested session
• add timeouts

TODO:

• check mdns is up to it (timing of changed records)
• think about multiple guest sessions on the same server (flow slightly different)