[WIP] UserAgent fixes

As discussed with @kibje at LinuxTag2014, we need to fix XBMC useragent to allow automatic detection of OS by various web tools. 
With this patch XBMC will try to imitate Chromium identification of OS.
I need some help from darwin developers as darwin code was blind-edited. @jmarshallnz @davilla @Memphiz ?
Still need to fix Linux, Android and FreeBSD.