[RFC] Avoid printing an error response when detecting xclip.

While we're at, using the slightly more portable hash technique to
detect the executable.  Also, there's no need to use `io.popen()` if we
aren't going to record the path.  Instead, let's use the simpler
`os.execute()` to detect the presence of xclip.
`command -v xclip >/dev/null 2>&1` is even more (POSIX) portable. 

Details: http://stackoverflow.com/a/677212/152142