RACE: pacemaker defaults and rules

[deurk]

A couple things are still to be decided about the pacemaker function of the race mode:

• What race_headstart should be there by default and should it be locked if sv_cheats is off?
• What to do with k_race_pace_legal?
• What to do about recording of demos?

Also, should ezquake automatically remove pacemaker from demos during playback (or based on cvar preference)