Currently if socket() or connect() syscall-s failed, parasite cures itself,
but criu has not got any signals and waits on accept().

This patch adds a futex to synchronize parasite and criu. The server socket
is created with SOCK_NONBLOCK and waits on the futex when a parasite
connects to it, only then criu calls accept() and it returns immediately.
Reported-by: Yohei Kamitsukasa <uhoidx@gmail.com>
Cc: Yohei Kamitsukasa <uhoidx@gmail.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>