lei_store: keyword extraction from mbox and Maildir

lei_store: keyword extraction from mbox and Maildir

Dovecot, mutt, and likely much other software support mbox
Status/X-Status headers.  Ensure we have a way to extract these
headers as JMAP-compatible keywords before removing them for git
storage.

->add_eml now accepts setting keywords at import time,
and will probably be called like this:

	$lst->add_eml($eml, $lst->mbox_keywords($eml));
	$lst->add_eml($eml, $lst->maildir_keywords($fn));