lib-storage: Moved dbox-sync-rebuild API to more generic index-rebuild.

lib-storage: Added support for saving mail attachments separately via filesystem API. Currently this works only with sdbox and mdbox backends.

configure: Removed --enable-header-install. It's now always enabled. There didn't seem to be much point in keeping it optional anymore. They're now more useful than before because shared libraries are also installed. --HG-- branch : HEAD

Split dbox (single-dbox) and mdbox (multi-dbox) into separate storage backends. This cleans up the code, makes it faster and also fixes some bugs. Super-fast maildir migration code was also dropped, at least for now. --HG-- branch : HEAD