Instead of exporting the C function mygetgrouplist as a global symbol to
conflict with other symbols of the same name, use trivial Go code and a
static C function.

Change-Id: I98dd667814d0a0ed8f7b1d4cfc6483d5a6965b26
Reviewed-on: https://go-review.googlesource.com/23008
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>