Add validation to check for invalid ranges in the form of 123.4.5.6-123.4.5.10.

I've had issues where someone has entered what they thought was a valid range, along the lines of 123.4.5.6-123.4.5.10, when they should have been entering 123.4.5.6-10. It's an easy mistake to make and has cropped up a couple of times here recently. The current form validation lets that format through, but it ends up causing unexpected behavior when the IP login is being processed.

I'm attaching a patch to block submission of those ranges. While I was at it, I also rewrote the existing validation, to use a regular expression as was suggested in a @todo comment.