qa: build unshared nfct environment

nssocket forks and change netns pre-establishd by ip(8), serves its
socket descriptor to parent via nssocket().  Since this socket is
isolated, it can be used to create regression tests for conntrack.

This also adds a conntrack event testcase as a first user.
A ct_echo_event.sh script is provided to build and run this test
automatically:

  # ./qa/ct_echo_event.sh
  make: Entering directory...
  ...debug output like:
      [NEW] tcp      6 2 SYN_SENT src=10.255.255.249 dst=10.255.255.250 sport...
   [UPDATE] tcp      6 2 SYN_RECV src=10.255.255.249 dst=10.255.255.250 sport...
  ...
  [DESTROY] icmp     1 src=10.255.255.249 dst=10.255.255.250 type=8 code=0...
  # echo $?
  0

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