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
Fix WM_UNINITMENUPOPUP missing definition for VC6.

The previous workaround of defining it explicitly if it's not defined was left
in src/msw/frame.cpp while the code using WM_UNINITMENUPOPUP was moved to
src/msw/toplevel.cpp in r73562. Move the fallback definition there too.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73682 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775