Added wxFDIOHandler::IsOk() and use it with wxSocketImplUnix.

Added wxFDIOHandler::IsOk() and use it with wxSocketImplUnix.

This will allow to use the base wxFDIOHandler class only in GUI-specific
network code and this remove its dependency on wxSocketImplUnix. IOW it paves
the way for a proper solution of the problem fixed by r61336 without the hack
of r61335 which results in linking problems (which went undiscovered until now
but were, in fact, always present, i.e. r61335 couldn't work).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61685 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775