lib-storage: convert dbox-common to use container_of

lib-storage: Remove unnecessary mail_save_context.dest_mail==NULL checks It can never be NULL after the previous change: "lib-storage: Always create mail_save_context.dest_mail". The code removal in maildir_transaction_save_commit_pre() seemed potentially dangerous, but I don't think such code path is possible anymore. Also even if it is, it's probably fine since the mail_free() is called even earlier than before (although that itself might have been a problem). This also removes last traces of code that made it possible to save mails to mbox without assigning UID to the mail. The previous commit already caused this, so this is just removing dead code.

dbox: Optimize POP3 MAIL_FETCH_UIDL_BACKEND. We keep track of the highest UID known to have POP3 UIDL in index's header. If saving adds a newer message, it'll also update the header. When fetching UIDL_BACKEND, we can need to check only mails with lower UIDs. There are some race conditions here, but normally UIDLs are set only once during migration so it shouldn't matter.

global: unsigned int:1 -> bool:1 perl -i -pe 's/unsigned int ([^,:;]+):1;/bool $1:1;/' **/*.[ch]

dbox: Don't cache pop3.uidl|order unless index header indicates there are those. They exist only when doing a migration, so it's pretty wasteful storing "doesn't exist" for all other installations.

lib-storage: Code cleanup: dbox_save_context.cur_file doesn't need to be in dbox-common.

Marked functions parameters that are allowed to be NULL. Some APIs were also changed. The non-obvious APIs where NULL parameter was changed to "" are master_service_init() and auth_master_user_list_init(). These checks can currently be enabled only on a patched clang: http://llvm.org/bugs/show_bug.cgi?id=6786

Moved GUID code to liblib. Use guid_128_t type consistently everywhere.

dbox: Figure out internally when message's "physical size" doesn't match "written size". --HG-- branch : HEAD

sdbox: Copying is now done with hard links. --HG-- branch : HEAD

dbox: Allow plugins to change output stream while saving messages. --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