Improve when waypoints are shown in ka/tka/kh/ctf

Improve when waypoints are shown in ka/tka/kh/ctf

Hid the waypoint from enemies when the player is "invisible."
Defined IS_INVISIBLE as alpha <= 0.25, has Invisibility powerup, or Cloaked mutator enabled, in qcsrc/server/utils.qh.
Showed waypoints in warmup in ka/tka/kh, and to spectators (unless they're spectating a key/ball carrier, so the waypoint isn't constantly shown at the top of the screen).
Allowed g_balance_keyhunt_delay_tracking to be negative, so that the tracking waypoints never appear.
Added cvars g_keepawayball_tracking and g_tkaball_tracking to customize when waypoints show in ka/tka.
- 0 means they never show.
- 1 means they always show.
- 2 means they show only on the dropped ball.