unshare: allow persisting mount namespaces

unshare: allow persisting mount namespaces

We can create a reference (bind mount) to the new namespace after
unshare(2), but it does not make sense to do it within unshared
namespace. (And if I read kernel fs/namespace.c: do_loopback()
correctly than copy mount bind mounts of /proc/<pid>/ns/mnt between
namespaces is unsupported.)

This patch bypass this problem by fork() where parent continue as
usually (call unshare(2), setup another things, etc.), but child
waits for /proc/[ppid]/ns/mnt inode number change (the ino is
changed after parent's unshare(2)) and then it bind mounts the new
namespaces and exit.

Signed-off-by: Karel Zak <kzak@redhat.com>