`report-emacs-bug' prompting rework

[Lars Ingebrigtsen]

As previously discussed (like, a year ago), the prompting sequence when
sending bug reports, especially from "emacs -Q", was kinda
unsatisfactory.  (The main point was that it required that you had a
valid-ish From header, even though you might end up sending the message
through mailclient.el, which will then just rewrite your From header,
anyway.)

Fixing this required a bit of refactoring of the sendmail.el/Message
setup functions, but hopefully I didn't screw up anything too badly.

And the prompting sequence looks OK to me now, but please let me know if
anything is wonky.

-- 
(domestic pets only, the antidote for overdose, milk.)
  http://lars.ingebrigtsen.no  *  Sent from my Rome