Implemented RFC2919, the 'List-ID' field for mailing

lists.  This is now a top-level Citadel header field (L), which makes
it parseable by the Sieve engine.  Also added List-ID to the header
fields selectable in the mailbox filter rules editor.  And of course,
we are generating this field when we send out traffic from a Citadel-hosted
mailing list.  This also closes Bug #371.