iptables: add connmark savedscp support

iptables: connmark - add savedscp option

Naive user space front end to xt_connmark 'savedscp' option.

e.g.

iptables -A QOS_MARK_eth0 -t mangle -j CONNMARK --savedscp-mark 0xfc000000/0x01000000

Will save DSCP into the top 6 bits and OR 0x01 (ie set) the least
significant bit of most significant byte.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>