os/user: add LookupGroup, LookupGroupId, and User.GroupIds functions

As part of local testing with a large group member list, I discovered
that the lookup functions don't resize their buffer if they receive
ERANGE.  I fixed this as a side-effect of this CL.

Thanks to @andrenth for the original CL.

Fixes #2617

Change-Id: Ie6aae2fe0a89eae5cce85786869a8acaa665ffe9
Reviewed-on: https://go-review.googlesource.com/19235


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>