qa: build unshared nfct environment

nssocket forks new netns child and set network environment defined in
nssocket.h

    * connect parent by veth peer
    * device names and addresses are VETH_...
    * set default gateway device to dummy, named DUMMY_LINK_NAME

It enables get socket fd from child ns by nssocket(), see example
ct_tcp_event.c

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
conntrack: add mark event filter

This patch adds mark filter for event listener, using same struct
nfct_filter_dump_mark.

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
xcnt: add XCOUNTERS

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
Merge branch 'master' into testing