Suppress error output from Unix system information functions.

Redirect stderr to /dev/null in wxGetCommandOutput() to avoid errors from the
shell if the command we're trying to run doesn't exist.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73681 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775