Afaict, userns_exec_1() is only used to operate based on privileges for the
user's own {g,u}id on the host and for the container root's unmapped {g,u}id.
This means we require only to establish a mapping from:
- the container root {g,u}id as seen from the host -> user's host {g,u}id
- the container root -> some sub{g,u}id

The former we add, if the user did not specifiy a mapping. The latter we
retrieve from the ontainer's configured {g,u}id mappings.

Closes #1598.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>