global: Fix spelling mistakes in comments Original work by @andreasschulze and @jsoref

sdbox: Don't log an error if stub is added twice There's no locking for them, so it's fine if two processes add the same mail. The second one could be ignored, but it was a bit easier to just let it rename over the first one.

dbox: Added asserts to make sure the mail body isn't tried to be read too early while saving. If the mail headers are still being written, the opening will just fail thinking the mail is corrupted.

dbox: Reverted the "check if altpath is mounted" change for now. If alt path didn't yet exist, it reported bogus errors. It should somehow check if there is anything under the alt root dir.

dbox: If alt path is lost, fail with an error rather than rebuilding indexes. This could be simply because alt storage is unmounted at the time and rebuilding would just lose messages. If this error happens unintentionally, it's still possible to fix it with "doveadm force-resync".

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

dbox: dbox_file_move() was used by only single-dbox, so moved it there. --HG-- branch : HEAD

sdbox: Copying is now done with hard links. --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