Following up on a discussion in de.comm.software.gnus I'd like to
propose a change in the handling of `M-x customize-*' buttons.
Currently `customize-apropos' is called on any such string.  This may
result in a little bit unexpected behavior.  Check for example the
result of `M-x customize-face RET highlight RET'.

In my opinion it would be better if the given function were called
exactly instead of `customize-apropos'.  The attached patch
accomplishes this.  The patch also removes support for "custom:" and
"custom:/" type urls which I've never seen before.  Of course this is
discussable.  Keeping it included would either require two separate
functions for handling the customize stuff or a conditionalization
inside of `gnus-button-handle-custom'.  WDYT?

Here is the patch: