extensions: libxt_TOS: Add translation to nft

extensions: libxt_TOS: Add translation to nft

Add translation for TOS to nftables. TOS is deprecated
ans DSCP is ued in place of it. The first 6 bits of
TOS specify the DSCP value.

Examples:

$ iptables-translate -t mangle -A PREROUTING -p TCP --dport 22 -j TOS --set-tos 0x10
nft add rule ip mangle PREROUTING tcp dport 22 counter  ip6 dscp set 0x04

Signed-off-by: Gargi Sharma <gs051095@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>