If a Mac user has more than 256 groups, getGroupList returns -1 but
does not correctly set n. We need to retry the syscall with an
ever-increasing group size until we get all of the user's groups.

The easiest way to test this change is to set n to a value lower than
the current user's number of groups, test on a Mac and observe
a failure, then apply the patch and test that it passes.

Fixes #21067.

Change-Id: I0f5c4eac1c465226a460bc0803eff791dcfd4200
Reviewed-on: https://go-review.googlesource.com/51892Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>