machine: enter target PID namespace when adding a live mount

machine: enter target PID namespace when adding a live mount

machinectl fails since 21935150a0c42b91a322105f6a9129116bfc8e2e as it's now
mounting onto a file descriptor in a target namespace, without joining the
target's PID namespace.
Note that it's not enough to setns CLONE_NEWPID, but a double-fork is required
as well, as implemented by namespace_fork().

Add a test case to TEST-13-NSPAWN to cover this use case.

(cherry picked from commit 98f654fdeab1e1b6df2be76e29e4ccbb6624898d)

Gbp-Pq: Name machine-enter-target-PID-namespace-when-adding-a-live-mou.patch