This implements the algorithm proposed in
https://github.com/containers/storage/issues/852#issuecomment-798954173,
which is:
1. find available IDs from subuid/subgid file; by subtracting the used
IDs (from other containers) as well as additional IDs, we get the IDs
available to allocate;
2. target ID range is [0, requestedSize), subtract the additional IDs;
3. allocate IDs from range in step 1; the number to allocate is the
number of IDs in step 2;
4. generate a mapping from IDs in step 3 to the ones in step 2.
Closes: https://github.com/containers/storage/issues/852
Signed-off-by: Kan Li <likan@google.com>
Kan Li