lib-storage: Fix error handling when sorting mails. All errors were treated the same as if message had been expunged. This caused potentially wrong results to be sent to the client without any indication that they're wrong.

lib-storage: Search code cleanup.

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

mail_index_lookup*() can't fail anymore. Changed several APIs not to return failure anymore. --HG-- branch : HEAD

Changed mail-storage API to do the mail sorting internally. Optimized it internally to keep a 32bit sort_id field in index for each used primary sort condition. Practically this should mean less disk I/O, memory and CPU usage when SORT command is used. --HG-- branch : HEAD

mail-storage.h interface changes, affects pretty much everything. FETCH, SEARCH, SORT and THREAD handling were pretty much moved from lib-storage/ to imap/ so adding non-index storages would be much easier now. Also POP3 server can now be easily implemented with lib-storage. Not too well tested, and at least one major problem: partial fetching is _slow_. --HG-- branch : HEAD

SORT and THREAD didn't work with sequence numbers at all. SEARCH was also a bit broken with it if another session had deleted messages which we hadn't yet told to client. --HG-- branch : HEAD

THREAD=REFERENCES implementation. Doesn't crash, but I'm not sure how correct replies it produces :) --HG-- branch : HEAD

Naming style changes, finally got tired of most of the typedefs. Also the previous enum -> macro change reverted so that we don't use the highest bit anymore, that's incompatible with old indexes so they will be rebuilt. --HG-- branch : HEAD

Renamed IBuffer and OBuffer to IStream and OStream which describes their functionality better. I tried to keep the variable names and comments also sensible. --HG-- branch : HEAD

First implementation of SORT extension. String comparing still not up to spec, so we don't advertise it in capability string yet. The code supports getting the data partially pre-sorted to reduce memory usage and make it faster. So, in future we could use this by creating sorted binary trees. Also moved mail-storage-register.c into it's own .a lib to fix circular dependencies. --HG-- branch : HEAD