Basic API
---------
An article is opened with art_open() and closed with art_close()
There is no side-effect, all I/O is done via a t_openartinfo.
art_close() is not implied anywhere - you must call it when you need to.

t_openartinfo
-------------
This is basically an 'open article context'. You can pass these around or do
anything with them independently of the rest of the program.

This structure contains open file pointers, one to the raw article,
the other to the cooked article. There is no file related to these streams.
If you close them, the data is lost.

Each stream has a summary array of type t_lineinfo which is an array of
line offsets and 'hint' flags associated with that line.
The array for the raw stream is built on demand since most of the time it
is not needed.
The flags are used for speedy access to parts of the article as well as
information about interesting things on that line.

The cooked stream lineinfo is of size 'cooked_lines' which is found in
the t_openartinfo structure. The size of the raw stream is stored in
the article structure information attached to the header.

The header contains all the useful information about the article & its
structure.

t_header
--------
This builds upon the header structure in tin-1.4
The main structure consists of pointers to the regular RFC822 headers

Every article has a pointer to an 'ext' structure (of type t_part) which holds
extended header information.

If there are attachments, then the ext structure is the head of a linked
list of attachments.

t_part
------
There is one of these per article 'part' or attachment. The article as a
whole counts as 1 part so there will always be at least one of these structures
linked to the header.
Each t_part is pre-initialised to the RFC2045 defaults. (text/plain, 7bit etc..)
Content-* headers if present will then supersede this information.
The other fields are:
params		A linked list of parameters associated with the Content-Type and
			Content-Disposition headers
offset		The byte offset of this attachment. The offset of header->ext is
			the offset of the 822 body
lines		The number of lines of raw text in this attachment. The number of
			lines in header->ext is the total number of lines in the article
			body
uue			A linked list of uuencoded sections found in the current t_part
			The uue filename is found in a parameter called 'name'



The code tries to conform to the following RFC's

1806 - Content-Disposition header
2045 - MIME article extensions to RFC822
2046 - Content-Type, multipart article structure
2047 - MIME encodings (most of the code still refers to 1522)
