ZD_UpdateJoinLimit() function

Executing this function triggers the joinlimit, so players can't join anymore.

Requirements:
 - Survival gametype
 - sv_joinlimit must be greater than zero (any value but it must be set)
 - server-side only

int ZD_UpdateJoinLimit()

Return values:
 -1 = Joinlimit is not active.
  0 = Gametype is not Survival or not executed server-side.
  1 = Joining successfully blocked.
  2 = Joining already blocked.