Add post_id information to messages

This patch will add the value of mlist.post_id to the
mail during processing. It adds a X-Post-ID header,
which can then be searched for and placed in the
header/footer of the message (non-digest only). It
will also grab this information to put it in the
archives, so if you use a searchable archive you can
search for a message by post_id. This part is a little
'iffy'. The patch assumes you have the ht://Dig
patches installed, but should work fine without them;
will likely complain about templates/en/article.html
and templates/en/htdig-conf.txt

There is also added the ability to inflate the post_id
value from the "General" section in the GUI. It makes
sure you keep the value as a float, as well as only
lets you increase it. This is handy if you are
importing messages from an old archive and need to
start the post_id at a larger number so as not to
interfere with old messages.

Left to do as of this writing: i18n (somebody?),
documentation in Gui/General.py explaining post_id
better (and why you might want/need to mess with it),
documentation in Gui/NonDigest.py or
templates/en/headfoot.html explaining %(post_id)s variable.