uid-classification: properly classify *all* container UIDs

uid-classification: properly classify *all* container UIDs

A bit confusingly CONTAINER_UID_BASE_MAX is just the maximum *base* UID
for a container. Thus, with the usual 64K UID assignments, the last
actual container UID is CONTAINER_UID_BASE_MAX+0xFFFF.

To make this less confusing define CONTAINER_UID_MIN/MAX that add the
missing extra space.

Also adjust two uses where this was mishandled so far, due to this
confusion.

With this change the UID ranges we default to should properly match what
is documented on https://systemd.io/UIDS-GIDS/.