more seamless server upgrades

[totaam]

At present, when we upgrade a server, the client gets disconnected.

There are a number of things we can do to improve this:

• make the client re-connect automatically (gtk client: auto reconnect feature #3010, see also handle connection issues, session events, network drop outs, power events, etc #540)
• make the client freeze the windows until the new server remaps them
• make the old server pass the socket connections to the new server (so the client won't even notice the change at all)

[totaam]

Mostly done in the commit above.

Still TODO:

• more testing - especially on systems without a valid XDG_RUNTIME_DIR
• deal with ibus pid somehow - a new start variety perhaps? one that we kill on stop
• teach the server to tell clients that they should try to re-connect

[totaam]

Side-stepped the ibus issue by turning it into a daemon in 272358e.