jatom.js: fix 'link' population

Before this change, the logic for finding the 'link' for
an atom entry would just grab the first 'link'-type node
from a given entry.  But an entry may have multiple 'link'
nodes; one needs to grab the '<link rel="alternate" ....'
node.

 * http://www.xml.com/pub/a/2004/06/16/dive.html

   -> "Every Atom feed and every Atom entry needs an
       alternate link. It is the only type of link that
       is required by the Atom feed specification."

An example feed that uses multiple 'link' nodes (emitted
'link' field is wrong before this change):

 * http://blog.racket-lang.org/feeds/posts/default