Adding imap_rfc822_parse_headers(). This function lets you grab mail headers

Adding imap_rfc822_parse_headers(). This function lets you grab mail headers
from anywhere - a saved message in an archive, a pop server, a database, etc
- and parse them into an object similar to what imap_headerinfo() returns.
It's just missing the flag info and other things that only come from the
IMAP server.

In the process of doing this, I seperated some more code out into a support
function - _php_make_header_object() - since it was used in multiple
functions. The module is slowly getting well organized.