Don't declare things in more than one place. Move some declarations

between headers.  Code is now fully prototyped and compiles cleanly
with gcc -Wstrict-prototypes -Wmissing-prototypes -Wnested-externs
-Wredundant-decls.  Closes #723788.

include/ship.h

@@ -193,8 +193,6 @@ struct mchrstr {
 #define getshipp(n) \
 	(struct shpstr *) ef_ptr(EF_SHIP, n)
 
-extern double seagun(int effic, int guns);
-
 extern struct mchrstr mchr[];
 extern int shp_maxno;
 
@@ -216,8 +214,6 @@ struct mlist {
 #define SHP_FIR(b, t) (t ? (b * (logx((double)t, (double)60.0) < 1.0 ? 1.0 : \
 			     logx((double)t, (double)60.0))) : b)
 
-double logx(double d, double base);
-
  /* return codes from shp_check_nav */
 #define CN_NAVIGABLE	(0)
 #define CN_LANDLOCKED	(1)