os/user: add non-cgo versions of Lookup, LookupId

os/user: add non-cgo versions of Lookup, LookupId

If you cross compile for a Unix target and call user.Lookup("root")
or user.LookupId("0"), we'll try to read the answer out of
/etc/passwd instead of returning an "unimplemented" error.

The equivalent cgo function calls getpwuid_r in glibc, which
may reach out to the NSS database or allow callers to register
extensions. The pure Go implementation only reads from /etc/passwd.

Change-Id: I56a302d634b15ba5097f9f0d6a758c68e486ba6d
Reviewed-on: https://go-review.googlesource.com/37664
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>