Add process supervisor pkg for mananging and spawning multiple processes

This is still a work in progress.  This will be used to manage a long running docker init and act as pid 1 and a process supervisor for multiple processes.  

ping @creack  I'll probably need you to review the signal forwarding and reaping code, i think you already did this once before but I could not find the PR. 

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)