meta: Let user specify any combination of sreg/dreg

libnftables should not mask kernel errors. Let user specify any combination of
parameters and leave the error-checking to the kernel. The kernel will return
-EINVAL and users will know that they have to fix their code. This patch also a
removes a redundant variable that was passed to the snprintf-functions (flag).

A second iteration might be needed. I was not sure how to deal with
snprintf_default in the case of both sreg and dreg.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
expr: ct: Add support for setting the mark

This patch adds userspace support for setting properties of tracked connections.
Currently, the connection mark is supported. This can be used to implemented the
same functionality as iptables -j CONNMARK --save-mark.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>