This is done to speed up extraction significantly (both for portsnap
extract and update) in the case of slow NSS modules (like nss_ldap) as
it avoids having to look up uid and gid for root / wheel.
The reason this is a bigger problem for portsnap than for many other
system operations, is that portsnap executes tar(1) once for each port
so the internal uid/gid caching in tar(1) only helps a bit, resulting
in many user lookup calls.
Discussed with: cperciva
52e3f1458b