core: add namespace_fork() helper, that forks, joins a set of namespaces and forks...

core: add namespace_fork() helper, that forks, joins a set of namespaces and forks again

This helper is useful to ensure pidns/userns joining is properly
executed (as that requires a fork after the setns()). This is
particularly important when it comes to /proc/self/ access or
SCM_CREDENTIALS, but is generally the safer mode of operation.

(cherry picked from commit 27096982798e4f4d1498f9ce75c317b8d3376125)

Gbp-Pq: Name core-add-namespace_fork-helper-that-forks-joins-a-set-of-.patch