Use CF socket manager in GUI OS X applications.

Use CF socket manager in GUI OS X applications.

wxSocketManagerMac was never created under OS X since wxSocket code
refactoring as wxGUIAppTraits::GetSocketManager() wasn't overridden.

Doing this required an extra nasty hack with a global variable in the base
library which is used just to pass the socket manager pointer from the net
library to the core one without creating a dependency between them but this
seems unfortunately unavoidable.

See #11030.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61676 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775