tree-wide: use container_uses_namespace() in less trivial cases

tree-wide: use container_uses_namespace() in less trivial cases

In our current codebase we have a logical pattern:
list_empty(&handler->conf->id_map)
*IF AND ONLY IF*
container does NOT use user namespace

Which is perfectly correct nowadays, but once we (hopefully)
get an "isolated user namespaces" stuff ready it won't be the case.
It will be perfectly fine to have a user namespace with empty
/proc/*/{u,g}id_map files. Nowadays it's also possible,
but this kind of a configuration close to useless and nobody
actually uses it.

No functional changes intended.

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>