Extend the common header of struct empobj to include uid

Make sure all members of unit empobj_storage share uid in addition to
ef_type.

Add matching uid member to struct gamestr, struct natstr and struct
sctstr, and set them.

Swap struct empobj members uid and own to make that easier, and update
struct comstr, struct lndstr, struct lonstr, struct loststr, struct
nwsstr, struct nukstr, struct plnstr, struct realmstr, struct shpstr,
struct trdstr, struct trtstr accordingly.

Note that the uid isn't properly set for struct nwsstr, struct lonstr,
struct trdstr, struct comstr and struct loststr.  To be fixed.

include/plane.h

@@ -46,8 +46,8 @@
 struct plnstr {
     /* initial part must match struct empobj */
     short ef_type;
-    natid pln_own;		/* owning country */
     short pln_uid;		/* plane unit id */
+    natid pln_own;		/* owning country */
     coord pln_x;		/* plane x-y */
     coord pln_y;
     signed char pln_type;	/* index in plchr[] */