pre-dump: Collect mount info, root and nsmask

Well, we want to pre-dump files (fsnotifies), for that we
will need mountinfo-s and root, and for the latter -- the
current ns mask.

The problem with current ns mask is that its generation is
incorporated into ns IDs generation and dumping. And since
the ids dumping is not performed on pre-dump, let's just
provide a helper for ns-mask generation.

Strictly speaking, the whole ns-mask idea is not great, but
it's to be fixed later.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>