New flamethrower/flamethrower tweaks.

New weapon is noncraftable and appears in military and military_hw spawn
groups. Uses a new NO_BOOM flag to suppress the normal FLAME effect of
its ammunition and use a new FIRE_20 flag to make firing cost 20 charges
per shot.

Gives the vanilla spawn-only flamethrower a much-needed range boost to
make it more competitive with the craftable flamethrower_simple.

Also grants INCENDIARY flag to gasoline and expands FLARE to work as a
weapon flag in addition to as an ammo flag.