Fixed error messages and added 3 cvars to control damage

Fixed error messages in the error logs, about Get*ObserverTarget
complaining about players being not in game.
Added redie_ghost_world_damage (by default 0) to enable or disable damage
for ghosts by world elements (entities such as trigger_damage).
Added redie_ghost_player_damage (by default 0) to enable or disable damage
for ghosts by players (this only applys to knifes and grenades, projectile
weapons are always blocked due to technical limitations).
Added redie_ghost_fall_damage (by default 0) to enable or disable fall
damage for ghosts.
Fixed another error with get*observtarget and added one more cvar

Fixed another little error with get*observtarget.
Added cvar redie_ghost_disable_triggers (by default disabled) to block
activation of trigger_multiples (touch, starttouch and endtouch) by
ghosts.