lib-storage: convert shared to use container_of

lib-storage: Move "shared" storage name to mail-storage-private.h where it can be used.

lib-storage: Fixed using "auto" driver with shared namespaces.

lib-storage: Fixed crashing on accessing shared mailboxes. --HG-- branch : HEAD

Avoid including mailbox-list-private.h everywhere. --HG-- branch : HEAD

Initial commit for making mail_storage:mailbox_list to be from 1:1 to n:n. This will allow namespaces to share the same storage and a single namespace to have multiple storages (multiple mailbox formats). Neither works currently, and this commit probably breaks some things. --HG-- branch : HEAD

mail_user_*() now handles home directory lookups when necessary. Namespace owner is now a pointer to struct mail_user rather than a string. --HG-- branch : HEAD

shared-storage.h doesn't require index-storage.h, don't include it. --HG-- branch : HEAD

shared mailboxes: Cleanups, fixes and optimizations. --HG-- branch : HEAD

Added initial support for shared mailboxes. Listing isn't supported yet. Each user gets their own hidden namespace created automatically when they're accessed the first time. --HG-- branch : HEAD