Added native OS file system copy to efile

[Blaine Whittle]

prim_file:copy has a comment that reads " xxx Should be moved down to the driver for optimization".  This patch does just that, however to ensure compatibility I haven't changed the existing implementation of prim_file:copy/3;  I've added two new functions prim_file:raw_copy/2, and prim_file:raw_copy/3 which perform the copy in the driver.

git fetch git://github.com/bwhittle/otp.git prim_file_copy