socket: increase socket buffers for restoring queues (v2)

Sizes of send and recv buffers are set to maximum to restore queues,
after that sizes of buffers are restored.

O_NONBLOCK is set to a socket to prevent blocking during restore.

#2411

v2: do the same for unix sockets.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>