[PATCH] Explicit Cache-Control for lucid.http static content

The Chrome web browser revalidates every resource if no explicit Cache-Control or Expires HTTP/1.1 header is sent. This makes the page loads appear to take a long time on pages with a few external resources, adding 300-500ms 
per item. This includes the XHR json responses that set page images, like wireless signal indicators and the like-- the images are revalidated on every XHR response. As an example, the Network -> Interfaces page generates 16 
requests to the lucid http server:

    Main HTML
    cascade.css
    xhr.js
    tabbg.png
    cbi.js
    loading.gif
    ethernet_disabled.png
    reload.png
    reset.gif
    edit.gif
    remove.gif
    add.gif
    bridge.png
    vlan.png
    wifi.png
    iface_status 

Of those, 14 should be pulled from cache but they are all valdiated. The lucid server returns the correct 304 (Not Modified) responses but it delays the apparent page load time because of the backlog it creates at the http 
server.

I would suggest setting explicit cache control on all files returned by the lucid http directory dispatcher. The "Expires" header is reportedly more widely supported, however this relies on the clock on the OpenWrt? system 
being accurate, which may not be the case. The "Cache-Control: max-age=" allows the server to set a timeout in seconds. I've included a patch that sets revalidate interval to 1 year, which is the value recommended by google.
Reference:  http://code.google.com/speed/page-speed/docs/caching.html

Note this could create an issue if there are luci application which are generating files which change that are being served by the lucid http DirectoryPublisher?. I'm not sure if there is anyone doing that. If needed, this can 
probably be created as an option to the DirectoryPublisher? config stanza for each vhost.

Finally, this only affects the Google Chrome browser, as both IE9 and Firefox seem to have their own revalidation interval in the absence of explicit cache control which may be based on the last modified time of the resource. 
Even in Chrome, this change doesn't take effect until the item is re-served with a 200 HTTP response so Chrome's cache should be cleared after this patch is applied. The patch can be extended to include cache control on 304 
responses, but I'd not worry about cluttering the code with it because the problem will solve itself once chrome redownloads the resource.

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