nspawn: move container wait logic into wait_for_container()

Move the container wait logic into its own wait_for_container() function
and add two status codes: CONTAINER_TERMINATED or CONTAINER_REBOOTED.
The status will be stored in its argument, this way we handle:
a) Return negative on failures.
b) Return zero on success and set the status to either
   CONTAINER_REBOOTED or CONTAINER_TERMINATED.

These status codes are used to terminate nspawn or loop again in case of
CONTAINER_REBOOTED.