Merge ipt_TOS into xt_DSCP.

Merge ipt_TOS (tos v0 target) into xt_DSCP. They both modify the same
field in the IPv4 header, so it seems reasonable to keep them in one
piece. This is part two of the implicit 4-patch series to move tos to
xtables and extend it by IPv6.
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>