getgrouplist is non-standard and has slightly different semantics on
each platform.  Darwin defines the function in terms of ints instead of
gid_ts.  Solaris only recently supported the call, so stubbing out for
now.

Fixes #14696
Fixes #14709

Change-Id: I5a44538d41594909efb6f3f9610c55d638c36757
Reviewed-on: https://go-review.googlesource.com/20348
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>