added some missing include for a5c32cff1f56afe6f0c6c70d91a88a7a8238b2d7
nspawn: print PID and show how to enter the namespace

systemd-nspawn will now print the PID of the child.
An example showing how to enter the container is added
to the man page.

Support for nsenter without an explicit command was
added in https://github.com/karelzak/util-linux/commit/5758069
(post v2.22.2). So this example requires both a new kernel
and the latest util-linux.