Avoid printing an error response when detecting xclip.

While we're at, using the slightly more portable `command -v` 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.
Merge pull request #1555 from jszakmeister/silent-xclip-find-executable

Avoid printing an error response when detecting xclip.