Users can now call "runsc wait <container id>" to wait on a particular process
inside the container. -pid can also be used to wait on a specific PID.
Manually tested the wait subcommand for a single waiter and multiple waiters
(simultaneously 2 processes waiting on the container and 2 processes waiting on
a PID within the container).
PiperOrigin-RevId: 202548978
Change-Id: Idd507c2cdea613c3a14879b51cfb0f7ea3fb3d4c
Lantao Liu
Kevin Krakauer