Remove unnecessary check in side_engine::available_for_user().

As a side effect, a network user is able to take a side with
"allow_player=no", if host has started the game with "--debug" command
line argument.
Fixed "current_player" being assigned incorrectly (mp::connect_engine).

1st case:
Overwriting "current_player" with default value produces an incorrect
side config and causes the server to not recognize the player if its
username is not the same as "current_player" value. This is not
desirable, since a different player might be assigned to an ex-reserverd
side or a player might just simply change his/her username.

2nd case:
Using default value for "current_player" if no player has taken that
side, results in setting wrong controller in case side was reserved for
host.