fix for erlang.el compilation stuff in Emacs 21

[Dave Love]

At least in Emacs 21, C-x ` won't find an error from a compilation in
the shell buffer because `compilation-setup' doesn't get called.  This
fixes it cleanly by using compilation-minor-mode and maintaining the
shell keybindings.  It shouldn't have any effect on XEmacs or Emacs
20, but I've only tested it in Emacs 21.  It's against version 2.4.12,
on top of my previous two patches.

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