bundle2: transmit capabilities to getbundle during pull
Bundle2 capabilities of the client are sent to the server in the bundlecaps
argument of `getbundle`.
<item>
<title>{bookmark|escape}</title>
<link>{urlbase}{url|urlescape}rev/{node|short}</link>
<description><![CDATA[{bookmark|strip|escape|addbreaks}]]></description>
<pubDate>{date|rfc822date}</pubDate>
</item>