[Mutt] #3064: Implement "show" menu to list various internal configs as a nicely paged menu

[Mutt <fleas@xxxxxxxx>]

#3064: Implement "show" menu to list various internal configs as a nicely paged
menu

 Running ':attachments ?' in mutt shows current attachment settings on
 stdout instead of inside mutt... which is ugly.

 The attached patch implements a "show" menu with a few "demo" views to
 show what this can be used for:

   * show attachments: Replacement for "attachments ?" (latter kept for
 compatibility)
   * show config [regex]: Show all config options matching regex (if
 omitted, regex defaults to ".", i.e. all)
   * show mailboxes: Show all known mailboxes in order they were defined
   * show spam: Show all currently configures spam recognition commands

 Except "config" these don't seem to be extremely useful. However, what
 comes to mind and why I took the prototype this far is to mostly prepare
 the code to be ready to implement this for hooks. For example, 'show
 folder-hook [folder-regex]' would be nice to have to know what folder-
 hooks mutt would execute in what order.

-- 
Ticket URL: <http://dev.mutt.org/trac/ticket/3064>