unshare: Add option to automatically create user and group maps

unshare: Add option to automatically create user and group maps

This option is designed to handle the "garden path" user/group ID
mapping:

- The user has one big map in /etc/sub[u,g]id
- The user wants to map as many user and group IDs as they can,
  especially the first 1000 users and groups.

The "auto" map is designed to handle this. We find the first map
matching the current user, and then map the whole thing to the ID range
starting at ID 0.

Signed-off-by: Sean Anderson <seanga2@gmail.com>