reword an error message
make the titlescreen phase more reliable for host.lua, join.lua
bugfix: fix an infinite loop in gui1 textbox when run with no gui

The infinite loop was that when the textbox has no space, we word
wrap a single character infinitely and run out of memory by
printing endlines.
try to make join.lua more robust against missed callbacks
give plugins a chance to run in gui1 dialogs when disp is faked

Otherwise they cannot use any of the dialogs and we cannot do
leader select when joining a game.

This commit also adds an "any_running" helper to plugins_manager.
add some mp debugging output
a playerid occupying a game slot should not block themselves

It seems that this is normally not an issue because of latency,
but if all processes are running headlessly on a single machine it
seems that this can become an issue and a player can block
themselves from joining a game somehow. It seems unlikely that this
commit can cause bugs, although its not certain.