don't overwrite "current_player" for ai sides

actualy this is a little dangerous becasue currently some code uses
"current_player" during the mp connect phase t know whether a client is
allowed for a side. TODO: fix that
more removal of "player_id"
use "reserved_for" instead of "current_player"

"reserved_for" is only used during mp_connect and is not persistent over
savegames. that name matches more to what it does.
add compabiltiy code
add comment
don't generate "user_description" on the host

the host it not capable to generate strings in the correct language.
("(Reserved for $playername)" was reolved on the host). Instead we now
generate the description on the clients.
dont overwrite "name" in mpconnect

and also remove unneeded code.
we not dont overwrite "name" (the name of the leader) in mp connect if
it given by the scenario.
We also fix a previous commit "don't generate 'user_description' on the
host" which caused that the AI algorithm wasn't shown to the clients.