Adding patch to allow for 500k baud rate with screen.

This patch is left over from a patch submitted to GNU Screen screen-devel list back in 2011. Somehow it never made it into screen... but I can't find information on why it hasn't been included.

There are a few situations where a 500k baud rate is pretty useful. If nothing else, a fairly large number of modern car's OBDII ports run at 500k baud. Without this patch, screen is unable to keep up with the OBDII port in logging situations.