List actions menu

This patch chain adds some ground for for RFC2369 List-* header parsing abstraction, then implements a new menu to drive list help/post/reply/subscribe/unsubscribe/archive options for a message, according to its RFC2369 headers.

Detailed notes:

As part of enhancing RFC 2369 support, we now add the <list-action>
function.  When <list-action> is invoked, a new menu opens showing all
available list actions specified by RFC 2369.  It might look similar to
this:

| q:Exit  r:Reply  p:Post  s:Subscribe  u:Unsubscribe
|
| Select a list action:
|
|             Help: mailto:listserver@domain.net?subject=help
|             Post: mailto:listname@domain.net
|            Reply: --
|        Subscribe: mailto:listserver@domain.net?subject=subscribe%20listname
|      Unsubscribe: mailto:listserver@domain.net?subject=unsubscribe%20listname
|         Archives: --

This displays the values of List-* headers from the current message.
The user may select one of these menu options using the usual menu
navigation, or by pressing a bound key (see help).  On doing so, if the
action is a mailto: URI, a new draft message is created as specified.

Most pertinently, this makes it easier for Mutt users to subscribe to or
unsubscribe from a list message that their mutt configuration does not
already know about.  However there's no good reason not to support all
List-* headers in the same general framework.

(RFC 2369 allows HTTP URIs in List-* headers. These are not presently
supported, but could be if mutt were to provide more general support for
opening URLs.)