reject: Move reject state from struct natstr to rejectstr

reject: Move reject state from struct natstr to rejectstr

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

Reject state changes rarely.  Rewriting it to disk on every nation
update and retransmitting it in every xdump nat is wasteful.

To avoid this waste, move reject state to its own struct rejectstr.

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>