[NETFILTER]: IPv6 capable xt_TOS v1 target

Extends the xt_DSCP target by xt_TOS v1 to add support for selectively
setting and flipping any bit in the IPv4 TOS and IPv6 Priority fields.
(ipt_TOS and xt_DSCP only accepted a limited range of possible
values.)

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>