modified Emacs mode

[Dave Love]

I sent changes to the Emacs mode to erlang-patches, but perhaps the
modified erlang.el is useful now.  It's currently at
<URL:ftp://dlpx1.dl.ac.uk/fx/emacs/erlang.el>.

The bulk of the changes are actually cosmetic commentary/doc fixes.
The significant ones make it play better with Emacs 21 with up-to-date
facilities.  In particular, support for compilation error messages
from an inferior erl (as opposed to batch compilation) works in the
released Emacs 21, and currently with the development Emacs.  It's
only tested with Emacs 21, but shouldn't have broken anything else.
I'm interested in reports of anything I've broken or anything else
that wants fixing.