Use "m_" prefix for wxExecuteData members

No real changes, just rename the members to use the standard prefix
which is more consistent with the rest of wxWidgets and also allows to
avoid both the hacks with "foo_" names for the arguments of some
functions that were used to avoid the conflicts with member "foo" and
at least one remaining shadowing warning for "exitcode".

src/unix/apptraits.cpp

@@ -40,7 +40,7 @@ int wxGUIAppTraits::WaitForChild(wxExecuteData& execData)
     // prepare to wait for the child termination: show to the user that we're
     // busy and refuse all input unless explicitly told otherwise
     wxBusyCursor bc;
-    wxWindowDisabler wd(!(execData.flags & wxEXEC_NODISABLE));
+    wxWindowDisabler wd(!(execData.m_flags & wxEXEC_NODISABLE));
 
     // Allocate an event loop that will be used to wait for the process
     // to terminate, will handle stdout, stderr, and any other events and pass