Used by the routine at UPDATE_SERF.

The serf's fire routine, called from UPDATE_SERF. Refuses outright if the weapon slot at $EA98 is occupied, so only one shot exists at a time -- that single test is the whole of the game's rate of fire -- or if IN_DOORWAY says the player is outside the room's walk area. With the way clear it makes the rising sweep and calls FIRE_WEAPON, then picks the sword's first angle from the velocity.