unshare: allow persisting namespaces

unshare: allow persisting namespaces

For nsenter(1) we already support namespace specification by file
(e.g. bind mount to namespace /proc/[pid]/ns/[type] file). For
example:

  # nsenter --uts=/some/path

This patch extends unshare(1) to setup the bind mount for specified
namespace, for example

  # touch /some/path
  # unshare --uts=/some/path hostname FOO
  # nsenter --uts=/some/path hostname
  FOO

Note that the problem is mount namespace, because create bind mount
to ns/mount file within unshared namespace does not make sense.

Based on patch from Lubomir Rintel <lkundrak@v3.sk>.

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