[bug-mailutils] Work on mailutils

[Alain Magloire]

Finally got some time to do some work.  Not to duplicate,
I will be working on the 
- pop3 code

See doc/texinfo/pop3.texi for API documentation.
Feedbacks welcome.

And hopefully:
- nntp support
- mbox code
- mh code
- maildir

The idea was to provide low level API  for the protocols(IMAP, POP, etc ..)
So users could have direct access the lowest level call,
whithout going through the mailbox_t framework.

The second stage will be to reachitect the mailbox_t, message_t, header_t etc ..
framework.