Use int instead of long to count people

Code dealing with counting people mixes int and long pretty
haphazardly.  Harmless, because practical populations fit into int
easily on any machine capable of running the server.  Clean up anyway.

src/lib/update/prepare.c

@@ -94,7 +94,7 @@ prepare_sects(int etu, struct bp *bp)
 }
 
 void
-tax(struct sctstr *sp, struct natstr *np, int etu, long *pop, int *civ_tax,
+tax(struct sctstr *sp, struct natstr *np, int etu, int *pop, int *civ_tax,
     int *uw_tax, int *mil_pay)
 {
     *civ_tax = 0;