Introduced accept_if_subject and accept_if_from

To white list entries based on their subject or from address. This is
implemented in an weird fashion trying to not break the rest of the
behaviour of listadmin (i.e. undo). It seems to work well.

You also profit from the ability to have these statements multiple
times in your config.

A few oddities: accept_if_* is stronger than discard_if_*. So if you
list the same expression as both, accept_if_from and discard_if_from,
the matching mail will be accepted in the end.

You will, however, get two notifications about this as well.

Example session:
fetching data for gtk-devel-list@mail.gnome.org ...

[1/2] ============== some-list@mail.example.com ===================
From:     foo@bar.com
Subject:  coolum@yatt.com.au
Reason:   Post by non-member to a members-only list            Spam? 0
Automatically discarded due to matching Subject
Matching RE: "@yatt.com.au"
Automatically accepted due to matching Subject
Matching RE: "@yatt.com.au"