wxSocket::Initialize() and Shutdown() are for main thread only.

wxSocket::Initialize() and Shutdown() are for main thread only.

Calling Initialize() from another thread could never work before but it wasn't
clear that this was the case so document it in the functions comments and
documentation now and add asserts checking that they are called from the main
thread only.

Also simplify the code as we don't actually need to do any reference-counting
here and a simple boolean flag indicating whether the sockets are initialized
is enough.

Closes #11119.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61985 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775