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.
