Invesitgate and use --pid=host

[tomwilkie]

Turns out you can use the host's process namespace apparently. This could mean no more /hostproc, and tracer might even run in a container.

[tomwilkie]

weaveworks/weave#578

[peterbourgon]

n.b. Docker 1.5+, maybe no problem for us, as we're surfing the bleeding edge...