archive: use util.opener when archiving files.
This simplifies the code a bit and makes it easier to support symlinks.
<item>
<title>#desc|strip|firstline|strip|escape#</title>
<link>{urlbase}{url}rev/{node|short}</link>
<description><![CDATA[#desc|strip|escape|addbreaks#]]></description>
<author>#author|obfuscate#</author>
<pubDate>#date|rfc822date#</pubDate>
</item>