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.

include/sect.h

@@ -214,4 +214,6 @@ struct sctintrins {
 
 extern struct sctintrins intrchr[INT_DEF + 2];
 
+extern int fort_fire(struct sctstr *);
+
 #endif