Improvements to player drop behavior

Players and bots now surrender when the controlling client disconnects. This may reduce the occurence of #3228, but probably doesn't fix it (we don't have a repro case).

Server tweaks:
* Prevent crash when a client drops (#3224).
* Server messages now show up as "Server" in the chat dialog.
* The oldest player in a dedicated server is promoted to admin when the old admin drops.