Always send nocache_headers() for admin-ajax.php. This prevents iOS from...

Always send nocache_headers() for admin-ajax.php. This prevents iOS from caching ajax calls. http://stackoverflow.com/questions/12506897/is-safari-on-ios-6-caching-ajax-results

Turn off multi selection uploads for mobile devices. Currently Plupload cannot upload multiple files in iOS Safari. Only the first file makes it to the queue. 

Empty wp on unload to work around caching in iOS Safari.

Props azaozz, miqrogroove, nacin

fixes #22552


git-svn-id: https://core.svn.wordpress.org/trunk@22872 1a063a9b-81f0-0310-95a4-ce76da25c4cd