lib-storage: Added mail_save_copy_default_metadata()

lib-storage: If disable_reflink_copy_to=TRUE, disable hardlink/refcount copying.

lib-storage: Mailbox deletion API changed. Mailbox deletion should now be free of race conditions. The actual file deletion code is now responsibility of mailbox_list backend. --HG-- branch : HEAD

Mail copying API changed to be based on save API. This allows extending it easily in future. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Added flags and keywords parameters to mailbox_copy() in case they're wanted to be changed while copying. --HG-- branch : HEAD

Major mail-storage API changes. It's now a bit cleaner and much more plugin friendly. Removed proxy_mailbox* stuff, they were difficult to use and there's now much easier way to replace them. --HG-- branch : HEAD

Changed mail saving API to be nonblocking. --HG-- branch : HEAD