tests/server/sockfilt.c: Fixed support for listening sockets

This commit fixes support for sockets that are ready to accept
a new connection and have previously been put into listening mode.

It also includes changes which are the result of investigation
regarding Windows STDIN. These changes are the preparation for further
improvements regarding support for reading data from STDIN on Windows.

Open issue: WaitForMultipleObjectsEx does not support PIPE handles
which are returned by GetStdHandle while running without a GUI.