current compile.el issues

[Dave Love]

The :link attribute of compilation-error-regexp-alist doesn't work in
an uninstalled Emacs since it looks for the file in `doc-directory'
and it's actually in `data-directory'.

Obviously developers don't agree with most of this, but I guess other
users and providers of add-ons will:

 * I think `compilation-context-lines' should default to 0, per
   previous behaviour.  That way you can tell what the current error
   line is.

 * I find the highlighting of output files distracting when looking
   for errors in the output, but `compilation-mode-font-lock-keywords'
   isn't customizable to turn it off.

 * Since (I guess) there's no way to limit the region parsed for
   errors now, C-x ` doesn't work the way it used to in Lisp
   *Compile-Log* buffers -- you get the first error in the buffer, not
   the first from the last compilation.  The lack of the
   `compilation-parsing-end' facility is also a serious annoyance for
   dealing with errors from evaluations in a Comint buffer.

 * Moving to errors in compilation-shell-minor-mode shifts point
   around so that it's no longer at the prompt -- I think it should
   just alter the window, if that's necessary to display the error
   message.

 * I still don't know what the proper definition of `informational'
   messages is, since messages can be highlighted with
   `compilation-info-face' when compilation-skip-threshold's value is 1.