On shutdown(), Windows can drop traffic still queued for sending even if that
wasn't specifically requested. As a result, fatal errors (those after
signaling which the server will drop the connection) were sometimes only
seen as "connection lost" on the client side, because the server-side
shutdown() erraneously discarded the correct error message before sending
it.

If on Windows, we now use the Windows API to access the (non-broken) equivalent
of shutdown().

Backport from trunk

include/violite.h:
  export mysql_socket_shutdown(). It lives in vio in the backport.
sql/mysqld.cc:
  Go through our own shutdown() rather than straight to the POSIX one.
vio/viosocket.c:
  Define mysql_socket_shutdown(). On UNIXoid systems, it's just a wrapper for shutdown(), but
  on Window, it uses DisconnectEx, which is magic.