intermission: store the current time to completed_time only on the first svc_intermis...

intermission: store the current time to completed_time only on the first svc_intermission-like command (Nexuiz switches to svc_intermission for final scoreboard display and to svc_finale for the voting screen);
intermission: in GAME_NEXUIZ, display standard scoreboard instead of intermission overlay, and allow showscores (TAB) to override svc_finale overlay
scoreboard: in intermission, show the time when the game ended, not the current time

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7370 d7cf8633-e32d-0410-b094-e92efae38249