The previous handling of <a> tags led to some less-than-ideal
layout in HACKING (most noticeable on a mid-sentence reference
to the valgrind home page).

* docs/hacking.html.in: Slight tweaks to <a> tags.
* docs/hacking1.xsl: Move <a> handling...
* docs/hacking2.xsl: ...here.
* HACKING: Regenerate.
Signed-off-by: NEric Blake <eblake@redhat.com>