Use int instead of long for military reserves

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

include/nat.h

@@ -105,7 +105,7 @@ struct natstr {
     int nat_timeused;		/* number of seconds used today */
     short nat_btu;		/* bureaucratic time units */
     short nat_access;		/* The tick when BTUs were last updated */
-    long nat_reserve;		/* military reserves */
+    int nat_reserve;		/* military reserves */
     long nat_money;		/* moola */
     time_t nat_last_login;	/* time of last login, 0 menas never */
     time_t nat_last_logout;	/* time of last logout, 0 means never */