A socket is in one of half-closed states, if it sent a fin packet
or it received a fin packet.

CRIU plays with fin packets to restore half-closed states too.

When we need to sent a fin packet from a socket, we can call
shutdown(SHUT_WR). When a fin packet has to be restore in
a received queue, criu generate a fin packet and send it via
a raw ip socket.

A raw packet is sent with the SOCCR_MARK mark to be able
to not block it.

v2: remove the SOCCR_FLAGS_ACKED_FIN flag
    introduce sets of bits for different actions with fin packets

travis-ci: success for series starting with [01/21] build: install libnet-dev
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>