src: add support for setting secmark

src: add support for setting secmark

Add support for new nft object secmark holding security context strings.

The following should demonstrate its usage (based on SELinux context):

    # define a tag containing a context string
    nft add secmark inet filter sshtag \"system_u:object_r:ssh_server_packet_t:s0\"
    nft list secmarks

    # set the secmark
    nft add rule inet filter input tcp dport 22 meta secmark set sshtag

    # map usage
    nft add map inet filter secmapping { type inet_service : secmark \; }
    nft add element inet filter secmapping { 22 : sshtag }
    nft list maps
    nft list map inet filter secmapping
    nft add rule inet filter input meta secmark set tcp dport map @secmapping

[ Original patch based on v0.9.0. Rebase on top on git HEAD. --pablo ]

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