Generic dispatcher

[Stefan Monnier]

For epa/eww/mhe/... I think we should introduce a generic dispatcher.
The way I see it, it would look like:

- M-x mail RET runs one of the mail backends.  First time around it
  prompts the user to choose a MUA, then stores the result in a Custom
  var and doesn't prompt any more.  This would rely on
  a mail-alternatives variable holding various MUAs such as Gnus, MH-E,
  ... and third party packages can easily register themselves in this
  list via their autoloads.  The user can reconsider his choice with C-u
  M-x mail RET in which case she gets prompted again.

- same for M-x irc RET
- same for M-x encrypt RET
- same for M-x browse RET
- we could push this further (e.g. auctex vs tex-mode.el) but I'm not
  sure it'd be such a great idea.


        Stefan