src: add ability to set/get secmarks to/from connection

src: add ability to set/get secmarks to/from connection

Labeling established and related packets requires the secmark to be stored in the connection.
Add the ability to store and retrieve secmarks like:

    ...
    chain input {
        ...

        # label new incoming packets
        ct state new meta secmark set tcp dport map @secmapping_in

        # add label to connection
        ct state new ct secmark set meta secmark

        # set label for est/rel packets from connection
        ct state established,related meta secmark set ct secmark

        ...
    }
    ...
    chain output {
        ...

        # label new outgoing packets
        ct state new meta secmark set tcp dport map @secmapping_out

        # add label to connection
        ct state new ct secmark set meta secmark

        # set label for est/rel packets from connection
        ct state established,related meta secmark set ct secmark

        ...
        }
    ...

This patch also disallow constant value on the right hand side.

    # nft add rule x y meta secmark 12
    Error: Cannot be used with right hand side constant value
    add rule x y meta secmark 12
                 ~~~~~~~~~~~~ ^^
    # nft add rule x y ct secmark 12
    Error: Cannot be used with right hand side constant value
    add rule x y ct secmark 12
                 ~~~~~~~~~~ ^^
    # nft add rule x y ct secmark set 12
    Error: ct secmark must not be set to constant value
    add rule x y ct secmark set 12
                 ^^^^^^^^^^^^^^^^^

This patch improves 3bc84e5c1fdd ("src: add support for setting secmark").

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>