Facebook URLs with exclamation marks (!) not accepted

I discovered this on one of my own sites; Facebook URLs sometimes contain embedded exclamation marks (bangs) which were causing a problem with the link parsing in links.inc.

The patch I'm attaching to this issue has been tested on a production site with over 10,000 rows in the links table, but I am still calling it "for review" just in case I've missed a problem. Please comment if it works or does not work for you, so I can get it committed ASAP.