unshare: Fix PID and TIME namespace persistence

unshare: Fix PID and TIME namespace persistence

After unshare(...) is called, /proc/self/ns/pid does not change.
Instead, only /proc/self/ns/pid_for_children is affected. So bind-mounting
/proc/self/ns/pid results in the original namespace getting bind-mounted.

Fix this by instead bind-mounting ns/pid_for_children.

[kzak@redhat.com: - add ns/time_for_children
                  - remove C++ comments
                  - resolve commit conflicts]

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
Signed-off-by: Karel Zak <kzak@redhat.com>