merge: tidy private data usage

 * rename Account to ConnAccount
 * add config Account
 * tidy notmuch
 * reorg notmuch functions
 * reorg pop functions
 * tidy pop
 * reorg imap functions
 * reorg maildir functions
 * reorg mbox functions
 * reorg nntp functions
 * tidy maildir
 * rename maildir_flags()
 * rename email include
 * rename Header to Email
 * Email doesn't begin with 'h'
 * notmuch: tidy data naming
 * notmuch: distinguish between Mailbox and Email data
 * notmuch: fix header references
 * nntp: tidy data naming
 * pop: tidy data naming
 * pop: separate email data
 * imap: tidy data naming
 * imap: more data tidying
 * change free functions to use void**