[cr_tests PATCH 1/1] ns_exec: define -g option to emulate ns cgroup

[Serge E. Hallyn]

Since I'm asking Oren to send out cr patches I suppose I should
really be doing the same things with cr_tests patches that I intend
to apply to git://git.sr71.net/~hallyn/cr_tests.git.

This one lets ns_exec be used to emulate the way ns cgroup auto-creates
cgroups, without having ns_cgroup mounted.  This will be used in
some of the tests, process-tree being first.