Napalm works again, which fixes incendiary grenades.

 * Stopped the fuel and n20 option, in spawn_air, from runtiming.
 * Added toxins as a spawn_air option.
 * Removed all use of the fuel option and replaced it with toxins, since fuel does not work with LINDA anymore.
 * Made spawn_air use flags and added HEAT, COLD, TOXINS, OXYGEN, CO2, NITROGEN, N2O, FUEL and AIR flags. You can use them in conjunction by doing "SPAWN_HEAT | SPAWN_TOXINS" for fire.