Use border-radius throughout core in addition to Mozilla and Webkit-specific properties

This is a followup to a comment @mfer made at #610234: Overlay implementation.

Why are mozilla and webkit specific border radius included and not border-radius itself? We should include that to be forward compatible.

The original comment applied to the overlay module, but there are other places in core that have this problem too. I think this patch gets them all.

Notes:
1. This patch entirely resulted from me acting like a monkey and copy-pasting the obvious things :) I haven't tested it one single bit.
2. In a couple places in themes/seven/style.css, the existing code had different border radius values for Mozilla and Webkit. I don't know why, so I just picked Mozilla to copy at random... someone might definitely want to take a look at those.