Use capability testing instead of UA sniffing

Use capability testing instead of UA sniffing

This should theoretically work fine, but I'm admittedly not *totally*
sure what this is supposed to do, so I haven't actually tested it.  The
old behavior was almost certainly wrong, though: is_khtml was returning
true for all WebKit, and non-ancient WebKit definitely supports
overflow-x.  Robert Stojnic/rainman should take a look at this to make
sure it's right, or anyone else who knows what this does.