make sure that wxProcess always have a valid PID set; add test unit for wxExecute,wxShell,wxProcess based on the code in the console sample; add some more infos/notes in wxProcess docs

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59784 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/utilsexc.cpp

@@ -899,6 +899,9 @@ long wxExecute(const wxString& cmd, int flags, wxProcess *handler)
     {
         // may be NULL or not
         data->handler = handler;
+
+        if (handler)
+            handler->SetPid(pi.dwProcessId);
     }
 
     DWORD tid;