fixup party full bell

after fix of bug #21578, the behavior from 1.10, in which game may not
start until all sides have chosen factions as needed, was restored.
we move the party full bell appropriately.
fix bug #18829

it turns out that when the "network_ai" controller type was introduced,
it was implemented on the server in the following way:

https://github.com/wesnoth/wesnoth/blob/a4f47a63c7b2c55fd7d8d3cd425ede256b39acdd/src/server/game.cpp#L464

change_controller requests to the type "ai" are always transformed
by the server into either "human_ai" for host or "network_ai" for client,
and in thunderstruck's refactor of the mp_connect dialogs, ai sides are
always set to "ai" in the WML sent to the server (and set this way on
the host).

however, because of code in play_campaign.cpp which long predated this
refactor, all sides on an mp client are either set to human (if that
player controls them) or "network", or "null".

this causes problems because if that player saves the game, the savegame
will record these sides at human controlled rather than ai controlled...
if the game is reloaded oos can occur, although I won't detail it here.

we update the play_campaign.cpp code to ensure that "ai" controller types
always resolve to human_ai or network_ai as appropriate. additionally,
we make sure that on the host, sides always resolve to human_ai rather
than ai. so the type "ai" should never be observed during an mp game,
and only during scenario configuration, and perhaps during replays.

additionally, we ensure in playturn.cpp that if the client gets a message
from the server to set a side to "ai" during the game, it will in fact
set it to "human_ai" or "network_ai", to preserve the invariant.

finally we also switch over observers to follow this behavior --
previously there was a hack here in the server which would make sure
that any observers which join see all sides as controlled by "human"

https://github.com/wesnoth/wesnoth/blob/a4f47a63c7b2c55fd7d8d3cd425ede256b39acdd/src/server/game.cpp#L197

in order that they do not attempt to substitute themselves for a player.

we change this by removing the hack, and programming the client to remember
if the player is an observer, and if so not attempting to substitute themself
at game start.
don't display network_ai sides as replacements for dropped sides

after 18829 fix, don't display "network_ai" sides as possible
controllers for a dropped side, as all ai sides now have this value.
don't show network_ai controllers in :give_control dialog
add debugging output to server
implement "last 5 chats" notification properly

(fixup 6d460b98271312f806fadfa037b7ea082e862a24)

proper code to find 5'th \n is:

		int endl_pos = -1;
		for (int ctr = 0; ctr < 5; ctr++)
			endl_pos = i->message.find('\n', endl_pos+1);

and not

		int endl_pos = 0;
		for (int ctr = 0; ctr < 5; ctr++)
			endl_pos = i->message.find('\n', endl_pos);