Add checks around kill

[cjyar]

If the program is configured to use kill to signal the sidecars to exit, then it should do some checks.

• On startup, it should check to see if it has shareProcessNamespace. It could try to check for compatible UIDs between its container and the sidecars, but it would be difficult or impossible to cover all the cases.
• It should ensure that hostPID isn't set. If this Pod started trying to kill all the processes on the node, bad things would happen.

Or, maybe we'll decide that kill is a bad idea and remove the associated code.