Introduce packet mark support with new ip,mark hash set. This includes
userspace and kernelspace code, hash:ip,mark set tests and man page
updates.

The intended use of ip,mark set is similar to the ip:port type, but for
protocols which don't use a predictable port number. Instead of port
number it matches a firewall mark determined by a layer 7 filtering
program like opendpi.

As well as allowing or blocking traffic it will also be used for
accounting packets and bytes sent for each protocol.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>