contact: Move contact state from struct natstr to contactstr

contact: Move contact state from struct natstr to contactstr

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

Contact changes rarely.  Since we avoid unnecessary updates, it
doesn't change at all unless option HIDDEN is enabled.  Rewriting it
to disk on every nation update and retransmitting it in every deity
xdump nat is wasteful.

To avoid this waste, move contact state to its own struct contactstr.

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>