waitpid() on the child process when changing namespaces
waitpid() on the child process when changing namespaces

This allows upstart to wait for the process as if it was the same one called.
Otherwise, we lose the process id and upstart assumes everything is over.
"expect fork" does not handle this properly.

TEST=manual check: /sbin/minijail --namespace-vfs -- /bin/bash -c 'sleep 1'
BUG=none

Review URL: http://codereview.chromium.org/1570004

omegaup/minijail,omegaup/minijail,omegaup/minijail,omegaup/minijail