Allow better customization of smoketrails

Instead of just "on"/"off", the "smoketrails" setting in doomretro.cfg
can be any combination of the following flags:
player = smoke trails for player rockets
revenant1 = smoke trails for revenant non-homing rockets
revenant2 = smoke trails for revenant homing rockets
cyberdemon = smoke trails for cyberdemon rockets
The default is "player|revenant2|cyberdemon".