os/user: add Go implementation of LookupGroup, LookupGroupId

os/user: add Go implementation of LookupGroup, LookupGroupId

If cgo is not available, parse /etc/group in Go to find the name/gid
we need. This does not consult the Network Information System (NIS),
/etc/nsswitch.conf or any other libc extensions to /etc/group.

Fixes #18102.

Change-Id: I6ae4fe0e2c899396c45cdf243d5483113932657c
Reviewed-on: https://go-review.googlesource.com/33713
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>