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.

test/functional/shell/viml_system_spec.lua

@@ -24,9 +24,7 @@ end
 local xclip = nil
 do
   if os.getenv('DISPLAY') then
-    local proc = io.popen('which xclip') 
+    xclip = (os.execute('command -v xclip > /dev/null 2>&1') == 0)
-    xclip = proc:read()
-    proc:close()
   end
 end