common: verify unix sockets support on Windows

common: verify unix sockets support on Windows

Windows Server 2016 doesn't support unix sockets. We'll
check the Windows version and log a message (but not at
error level) when attempting to initialize the admin socket.

This will prevent the following error from being printed
whenever mapping RBD images:

  AdminSocketConfigObs::init: failed: AdminSocket::bind_and_listen:
  failed to bind the UNIX domain socket to
  'C:/ProgramData/Ceph/out/client.admin.2560.asok':
  (10050) Unknown error

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>