netfilter/ct: support optional CTA_TIMESTAMP attribute

netfilter/ct: support optional CTA_TIMESTAMP attribute

Recent kernels support conntrack time stamping, which is a helpful
feature to determine the duration of a flow without building a flow
cache in your user space application, just to keep the 'start' time of
your flow.

Timestamps are recorded with nanosecond resolution once this feature
is enabled.

This patch adds optional support for the CTA_TIMESTAMP, then
modifies the dump routine to write that info in a format similar
to /proc/net/nf_conntrack.  This is an example output when using
NL_DUMP_LINE:

  udp 10.128.128.28:56836 <-> 10.128.129.255:8612 delta-time 30

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Thomas Graf <tgraf@suug.ch>