util: Add CopyFile helper

Copy file under both Posix systems and Windows.

Signed-off-by: Asias He <asias@cloudius-systems.com>
util: Move termios_windows.go under util

Also, move it to the util package.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Merge pull request #61 from asias/util

util: Add CopyFile helper