nanobsd: Add uname/gname to the spec

Also include the default NanoBSD uname/gname in the specification file.

By default, NANO_DEF_UNAME and NANO_DEF_GNAME map to root and wheel
respectively.

Reviewed by: imp, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48781