merge: upstream autocrypt feature

 * address: add mutt_addr_to_intl()
 * build: add autocrypt option
 * Initial autoconf and makefile setup for autocrypt
 * Add autocrypt config vars
 * Add database and schema initialization
 * Start autocrypt gpgme
 * Factor out mutt_edit_address() in send.c
 * Add initial autocrypt account setup
 * Add autocrypt header parsing to mutt_parse_rfc822_line()
 * Process autocrypt headers
 * Convert to use sqlite3_prepare_v3()
 * Add gossip header processing
 * Change autocrypt_db normalization to return struct Address
 * Change gossip header address comparison to use normalized addresses
 * Add another struct Email security bit for autocrypt
 * Add autocrypt line to the compose menu
 * Autocrypt outgoing emails
 * Fix process_gossip_headers() to look at the right envelope
 * Slightly improve the gossip header key update
 * Handle autocrypt message decryption
 * Print an error message if an autocrypt key can't be found
 * Add autocrypt source files to POTFILES.in
 * Add L10N translation message comments
 * Add AUTOCRYPT header to imap fetch list
 * Add prefer-encrypt prompt during account creation
 * Add account->enabled checks
 * Improve gen-map-doc work on multi-word maps
 * Convert peer_update to just pass the peer object
 * Basic autocrypt account menu
 * Add more translation string comments for autocrypt
 * Add option to scan mailboxes during first run
 * Add documentation for the autocrypt feature
 * Add autocrypt README file
 * Don't try to decrypt autocrypt messages if the option is off
 * fix test cases
 * iwyu: tidy #includes
 * rename local variable rv to rc
 * doxy: add comment blocks
 * rename local variable hdr to e
 * rename local variable msg to e
 * add braces to unclear 'if' statements
 * explicitly check retval for strcmp, buffer_len
 * add whitespace for clarity
 * tidy code
 * Add ability to create autocrypt account from an existing key
 * Automatically clear screen when popping the last menu
 * Add $autocrypt_reply
 * Allow nested setting of OptIgnoreMacroEvents
 * Turn off macro processing during autocrypt initialization
 * Enabled writing protected subject with autocrypt too
 * Minor autocrypt manual section updates
 * Add autocrypt documentation about shared key and keyring strategies
 * Add a note about autocrypt first-run and macros
 * Add L10N comment about autocrypt menu help line abbreviations
 * Mention base64 keydata being stored in the autocrypt database