Return the old file descriptor/pointer from wx(F)File::Detach().

Make Detach() more convenient to use.

Closes #15494.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74798 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/file.h

@@ -65,7 +65,7 @@ public:
 
   // assign an existing file descriptor and get it back from wxFile object
   void Attach(int lfd) { Close(); m_fd = lfd; m_lasterror = 0; }
-  void Detach()       { m_fd = fd_invalid;  }
+  int  Detach() { int fdOld = m_fd; m_fd = fd_invalid; return fdOld; }
   int  fd() const { return m_fd; }
 
   // read/write (unbuffered)