netfilter/ct: expand CT parameters that can be used in add/delete operations

This expands functionality for manipulating conntracks over netlink by
adding other attributes to nfnl_ct_build_message(). Added a command link
program to add conntracks.

https://github.com/thom311/libnl/pull/55

[thaller@redhat.com: cleaned up whitespace from original patch]
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>