[nmh-workers] added warning about post profile entry

[David Levine]

I just committed this:

- Added warning from all programs that read the profile if the profile
  contains a post entry, which is ignored, but does not contain a
  postproc entry.  In other words, if you get this warning and want
  to suppress it, your options include:
  1) Remove your post profile entry.
  2) Make your post profile entry a comment by prepending it with the #:
     comment indicator.
  3) Add a postproc entry that points to the post that you use.  That can
     be viewed with "mhparam postproc".

David