avoid needless Unicode<->MB conversions in Unix wxExecute(); simplify the code; provi...

avoid needless Unicode<->MB conversions in Unix wxExecute(); simplify the code; provide both versions taking char** and wchar_t** for compatibility; also use wxMacExecute() (renamed to wxMacLaunch() to avoid confusion) from all wxExecute() overloads but don't use it if wxEXEC_SYNC was requested as it doesn't support it

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@52722 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775