Customize how xrefs are followed in help-mode?

[Eric Abrahamsen <eric@ericabrahamsen.net>]

It's always bothered me that following a function/variable definition
button from a *Help* buffer goes to the file in a separate window -- I
almost always want to re-use the *Help* window.

Essentially, help-mode.el:209 hard-codes the use of `pop-to-buffer', I'd
like to be able to use `pop-to-buffer-same-window'.

From a user's standpoint, I guess what I'd prefer is to be able to give
a prefix arg to RET when following the button, but I don't see any way
of passing that arg down, plus there's no good equivalent for the mouse
click.

Would people consider a customization option to control this?

Eric