Fix for race from error on process start

This rather naively fixes an error observed where a processes stdio
streams are not written to when there is an error upon starting up the
process, such as when the executable doesn't exist within the
container's rootfs.

Before the "fix", when an error occurred on start, `terminate` is called
immediately, which calls `cmd.Process.Kill()`, then calling `Wait()` on
the process. In some cases when this `Kill` is called the stdio stream
have not yet been written to, causing non-deterministic output. The
error itself is properly preserved but users attached to the process
will not see this error.

With the fix it is just calling `Wait()` when an error occurs rather
than trying to `Kill()` the process first. This seems to preserve stdio.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

libcontainer/process_linux.go

@@ -84,6 +84,7 @@ func (p *setnsProcess) start() (err error) {
 		return newSystemError(err)
 	}
 	if ierr != nil {
+		p.wait()
 		return newSystemError(ierr)
 	}