Database synchronizaton for spamd/spamlogd

This adds an HMAC protected synchronization protocol for use by spamd and
spamlogd.

- spamd can receive updates from other hosts for GREY, WHITE, and TRAPPED db
entries, and will update the local /var/db/spamd accordingly.

- spamd can send updates when it makes changes to the GREY or TRAPPED
entries in the db to other hosts running spamd. (Note it does not send
WHITE entries because the other spamd will see the GREY changes and have
complete information to make appropritate decisions)

- spamlogd can send updates for WHITE db entries that it performs on the local
db to other hosts running spamd, which will then apply them on remote hosts.

note that while this diff provides synchronization for changes made to the
spamd db by the daemons, it does *not* provide for sychonizing changes
to the spamd db made manually with the spamdb command.

Synchronization protocol and most of the work by reyk@,
with a bunch of the spamd, and spamlogd stuff by me.

testing mostly at the U of A, running happily there under big load.

ok reyk@ jmc@

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