lib-imap-storage: Added imap_msgpart_contains_body()

imap: FETCH BODY.PEEK[HEADER.FIELDS (..)] didn't set wanted_headers optimization. Primarily this fixes imapc's prefetching.

lib-imap-storage: Added imap_msgpart_get_partial_size() missing from previous commit.

lib-imap-storage: imap_msgpart_size() should return size as uoff_t, not size_t

lib-imap-storage: Added support for getting BODYPARTSTRUCTURE Based on patch by Stephan Bosch.

imap_msgpart_parse() doesn't really need a mailbox parameter.

IMAP BINARY extension supports now FETCH BINARY command.

lib-imap-storage: imap-msgpart rewrite and API change. The new API allows first parsing the validity of section strings and later relying on them being valid without having to re-parse it. The implementation also fixes a few things and adds "partial fetch cache".

Error handling API changes to previous IMAP URL related changes.

Created lib-imap-storage for IMAP-specific functionality that uses lib-storage. This is done to prevent introducing dependencies on lib-storage in lib-imap. It contains IMAP message part access functionality, including URL-based access.