Mortgage rates in the U.S. are now at a 50-year low, creating a refinancing surge which is helping a vast number of financially-challenged homeowners lessen their borrowing cost. Freddie Mac, the government-run business which insures and buys mortgages said Thursday that the average 30-year fixed mortgage in the United States is now at 4.15 percent, a decrease from where it sat last week, at 4.32 percent....