Factor out common fortress fire code into fort_fire()

Factor out common fortress fire code into fort_fire()

This takes care of a number of bugs / inconsistencies:

* sb() fired support even when there were not enough mil.

* Shell resupply bugs: multifire() and quiet_bigdef() resupplied
  shells before checking all other requirements and could thus get
  more shells than actually needed.

Rename landgun() to fortgun() for consistency.