lib-mail: message_search_msg() passes through message_parser_deinit_from_parts()'s error string

lib-mail: Added message_search_more_get_decoded()

Replaced "decomposed titlecase" conversions with more generic normalizer function. Plugins can now change mail_user.default_normalizer. Specific searches can also use different normalizers by changing mail_search_context.normalizer.

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

Don't assert-crash on mail search if decomposed titlecase of search key is empty.

lib-mail: message_search_init() now converts search key to decomposed titlecase.

Changed header/body searching to do search key charset translation earlier. This cleans up and simplifies error handling.

lib-storage: mail_get_parts() no longer returns const pointer. The struct message_part contained context-pointer and other APIs were using it, so the pointer really couldn't be const without casting it away sometimes. --HG-- branch : HEAD

Removed pool parameter from message_search_init(). --HG-- branch : HEAD

message_search_more*() now returns bool instead of int. They can't fail. --HG-- branch : HEAD

message_search_more*() now returns bool instead of int. They can't fail. --HG-- branch : HEAD

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

Removed message body/header searchers. They're now combined into one message-search. --HG-- branch : HEAD