Clean up file headers.

This change rewrites and simplifies the copyright headers at the top
of all source files:

 * Remove "Emacs style mode select" line; this line was included in
   the headers for the originally released source files and appears
   to be to set the file type for old versions of Emacs. I'm not sure
   entirely why it was required but I don't think it is any more.
 * Remove "You should have received a copy of..." text from copyright
   header. This refers to the old 59 Temple Place address where the
   FSF headquarters used to be located and is no longer correct.
   Rather than change to the new address, just remove the paragraph
   as it is superfluous anyway. This fixes #311.
 * Remove ---- separator lines so that the file headers are barer
   and more simplified.