lib-mail: istream-attachment-connector now allows msg_size=-1 for "unknown".

Comment update.

lib-mail: Added istream-attachment-[connector|extractor]. istream-attachment-extractor can be used to parse messages and extract attachment MIME parts from them to wanted output streams. The attachments are base64-decoded if they can be later re-encoded to original input without any changes. istream-attachment-connector does the reverse by taking the base istream and attachment istreams and merging them together to create the original istream.