Fix using wxHTTP and wxFTP from worker thread in Unix ports.

This backports e18c8fd29a,
d421373c2e and
6c43aa90b6 from master to avoid crashes when
using wxHTTP or wxFTP from threads other than main.

See #17031.

include/wx/private/socket.h

@@ -308,6 +308,9 @@ public:
 protected:
     wxSocketImpl(wxSocketBase& wxsocket);
 
+    // get the associated socket flags
+    wxSocketFlags GetSocketFlags() const { return m_wxsocket->GetFlags(); }
+
     // true if we're a listening stream socket
     bool m_server;