Current for Plan 9 is implemented with /dev/user for
Uid/Gid/Username/Name, and $home environment variable for
HomeDir.

Implementing Lookup/LookupId is not done, which would
require parsing /adm/users. It is unclear of how much benefit
this would be.

R=golang-dev
CC=bradfitz, golang-dev, r
https://golang.org/cl/13203043