Rewrite wxExecute() implementation under Unix.

This commit changes wxExecute() to handle SIGCHLD to be notified about the
child process termination instead of detecting when the file descriptor
corresponding to the other end of a pipe opened in the parent process was
closed in the child as this was not reliable and could (and did) result in not
detecting the termination of the child processes that closed all their file
descriptors before exiting.

This commit also removes a lot of platform-specific code duplicating the
generic event loop sources support and reuses it for wxExecute() purposes too.

Final big change is that wxEndProcessData was merged into wxExecuteData and we
don't have two similar but quite different classes any more but just one,
which is used both to pass the information from wxExecute() to wxAppTraits
methods and to store this information until the child termination.

Closes #10258.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74350 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -574,6 +574,7 @@ All:
   event type constants to make using Bind() less verbose. E.g.
   wxEVT_COMMAND_MENU_SELECTED is now wxEVT_MENU (but the old name remains
   available for compatibility) (Catalin Raceanu).
+- Fix wxExecute() implementation under Unix (Rob Bresalier).
 - Add wxEvtHandler::CallAfter() method for asynchronous method calls.
 - Add support for symlinks to wxFileName (David Hart).
 - Add wxDIR_NO_FOLLOW flag for wxDir traversal (David Hart).