This avoids transferring all of /packages.json every time one reloads
the page.

* browse/web-interface.scm (request-packages-json-handler): Add
'request' parameter.  Pass it to 'request-file-handler'.
(request-file-handler): Add 'request' parameter.   Add a "Last-Modified"
header on responses, and honor "If-Modified-Since".
(request-handler): Adjust accordingly.