(EFF_OWNER): The assertion that EFF_OWNER implies ef_read() sets

player->owner is no longer used.  Use it to indicate that owner can be
accessed through struct genitem, similar to EFF_XY and EFF_GROUP.
(xdump): Fix ownership checking.  Old version worked only when
nxtitem() set player->owner.

(empfile): Set EFF_OWNER for EF_LOST.  This fixes `xdump lost'
disclosing other countries' losses.
(loststr): Document implications of EFF_OWNER.

(empfile): Set EFF_OWNER for EF_NATION.
(natstr): Rearrange members for EFF_OWNER, document.
(nat_ca): Unused so far.  Clean it up, add most missing selectors.
(fileinit): Use it.  This implements `xdump nation'.

(fileinit): Simplify setting map file size.

include/lost.h

@@ -35,8 +35,10 @@
 #define _LOST_H_
 
 struct loststr {
+    /* initial part must match struct genitem */
     short ef_type;
     natid lost_owner;		/* Who lost it */
+    /* end of part matching struct genitem */
     int lost_uid;		/* lost ID */
     char lost_type;		/* Type of thing (ship, plane, nuke, land, sector) */
     short lost_id;		/* ID of lost thing */