Refactor round handling in freezetag because I couldn't find any way to fix some...

Refactor round handling in freezetag because I couldn't find any way to fix some issues keeping the previous messy code (even if already a lot simplified in my previous work). Freezetag code now uses methods of the new interface round_handler instead of the code previously shared with arena and CA.

What's changed:
Countdown doesn't start anymore after warmup stage or if g_start_delay is 0 at the beginning of the game when there aren't enough players
Countdown to round start is now stopped if there aren't enough players
Weapons can't be used anymore when the round ends (it avoids to add accuracy stats)