Support for user and group ID mappings

Add options `--uid-map` and `--gid-map` to map multiple users between child and parent namespace: `bwrap --uid-map ' ...' ...`

Relying on shadow `newuidmap` utility, or implementing the same whitelisting logic from /etc/subuid (/etc/subgid).

This would allow for (non-fakeroot) chown calls in build containers.

Recently added in util-linux as `unshare --map-users=...`.