Backport of the 3.3dev fix to the 3.2 branch.
pr_url() and pr_boldurl() interpolated their argument directly into
<a href="$1">$1</a> without HTML escaping. The most notable caller
passes the raw HTTP Location: header from the scanned server, so a
malicious HTTPS target could inject arbitrary HTML/JS into an
operator's --htmlfile report. Route both the href attribute and the
link text through the existing html_reserved() escaper, matching the
pattern already used by every other pr_* HTML-output function.
Eric Gu