relations: Move relations state from struct natstr to relatstr

Relations state is relatively bulky: it's a big chunk of struct
natstr, and adds 200 bytes per country to xdump nat.

Relations change rarely.  Rewriting it to disk on every nation update
and retransmitting it in every xdump nat is wasteful.

To avoid this waste, move relations state to its own struct relatstr.

This is of course an xdump compatibility break.  We're not maintaining
xdump compatibility in this release.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>