lib-mail: Added HEADER_FILTER_CRLF_PRESERVE flag to istream-header-filter.

Removed CONTEXT_TYPE_SAFETY macro and reimplemented its functionality better. gcc/clang now gives a compiler error in many places if callback isn't exactly what was expected. It's also now much easier to add more of these checks.

istream-header-filter: Added i_stream_header_filter_add() to add extra headers.

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

istream-header-filter: Added HEADER_FILTER_END_BODY_WITH_LF flag. If body doesn't end with LF character, it adds it automatically.

header filter istream: Added HEADER_FILTER_ADD_MISSING_EOH flag to add end-of-headers line if it's missing. --HG-- branch : HEAD

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

Type safe callbacks weren't as easy as I thought. Only callback(void *context) can be handled generically. Others can be handled specially, but only if all the parameters are pointers, otherwise eg. int parameter can be replaced with long without compiler giving any warnings. --HG-- branch : HEAD

Added context parameter type safety checks for most callback APIs. --HG-- branch : HEAD

Added "bool" type and changed all ints that were used as booleans to bool. --HG-- branch : HEAD

Several size_t -> unsigned int changes. Try to keep "unsigned int" for counters, size_t for actual memory range sizes. --HG-- branch : HEAD

Changed istream-header-filter API a bit. Added HIDE_BODY option. Fixed mail.get_headers(). --HG-- branch : HEAD

header filter can now optionally use either LF or CRLF linefeeds. --HG-- branch : HEAD

istream-header-filter now parses the headers incrementally while read()ing, instead of doing it at initialization and storing into one large buffer. --HG-- branch : HEAD

We never do blocking reads/writes to network anymore. Changed imap and pop3 processes to use a single I/O loop. Not much tested yet, and currently LIST/LSUB may eat too much memory and APPEND eats all CPU. --HG-- branch : HEAD

Header caching redesigned. New design allows caching decisions per field, so they can be divided to temporary/permanent. Cached headers are now always returned in original order, old code didn't guarantee it. Some other caching changes. (still missing code to store changes in caching decisions) --HG-- branch : HEAD

Added istream-header-filter, which allows filtering specified headers from input stream. --HG-- branch : HEAD