Gnus Group Mail Spliting on mailing-list headers

Gnus Group Mail Spliting on mailing-list headers

* texi/gnus.texi: Document the new `list' split abbreviation and
`match-list' group parameter (bug#25346).

* lisp/gnus-mlspl.el: Use the `list' abbreviation when the new
`match-list' group parameter is set to `t'.
The split regexp is modified to match either `@` or `.` as domain
separator to comply with RFC2919 IDs too.

* lisp/nnmail.el: Add new `list' split abbreviation matching common
mailing-list headers.