Add `--user-parent` option to nsenter

Add `--user-parent` option to nsenter

When this option is used nsenter will fetch the parent user
namespace from any namespace file descriptors available.

It can be combined with existing `--user` option in which case
the parent user namespace will be fetched from the user namespace
and replace it.

The usecase of this option is when a user namespace that owns
the other type namespaces we want to switch to is not actually
bound to any process. Without using ioctl it is impossible to
acquire namespace file descriptor. For example, bubblewrap
`bwrap` command creates unbinded user namespace when `--dev`
option is used.