support using keep-id when only one mapping is available to the
rootless user.
When there is only one id available (e.g. there are no additional IDs
set in /etc/subuid and /etc/subgid for the unprivileged user), then
only add the identity mapping $ID -> $ID, leaving unmapped other IDs
in the user namespace.
[NO NEW TESTS NEEDED] it needs a configuration with only one ID
available.
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
83313c547c