Whitelist g_*_round_timelimit

Whitelist the round timelimit cvar so that servers can set round times other than 180 seconds (3 minutes) without being branded as modified.

This would allow having for example 2 minute round lengths which would waste less time for many people as they don't have to wait to respawn while 2 cowardly players refuse to move and find each other to finish the round or when some people refuse to kill anyone and only run away.