sometimes i grep "lambda", and would like to see the hits `next-error'
displays such that the hit is near the top of the window w/ the body (of
the lambda) following.  the attached diff adds this recentering capability,
and additionally makes it easier to tweak.

here are two equivalent customizations to demonstrate the change:

 cur: (add-hook 'next-error-hook (lambda () (recenter 2)) t)
 new: (setq next-error-recenter 2)

to change recentering behavior, the current method requires replacing the
anonymous function in `next-error-hook' with another one.  the new method
is less cumbersome.  on the downside, expanding `next-error' "API" can be
considered unnecessary feature creep.

here is a changelog entry:

	* simple.el (next-error-recenter): New defcustom.
	(next-error, next-error-internal): Recenter if specified,
	immediately prior to running `next-error-hook'.

below is the diff.  what do people think?

thi

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