namespace-util: introduce userns_acquire_self_root()

namespace-util: introduce userns_acquire_self_root()

This is a simple helper for creating a userns that just maps the
callers user to UID 0 in the namespace. This can be acquired unpriv,
which makes it useful for various purposes, for example for the logic in
is_idmapping_supported(), hence port it over.

(is_idmapping_supported() used a different mapping before, with the
nobody users, but there's no real reason for that, and we'll use
userns_acquire_self_root() elsewhere soon, where the root mapping is
important).