s3: use getgrset() when it is available

s3: use getgrset() when it is available

When getgrouplist() is not defined, use getgrset() if it is defined
instead of using the initgroups() + getgroups() combo

Major contributions from Yannick Bergeron <yaberger@ca.ibm.com>

Autobuild-User: Volker Lendecke <vlendec@samba.org>
Autobuild-Date: Sat Mar 19 10:09:38 CET 2011 on sn-devel-104
(cherry picked from commit ed46dfc4f16e230645fae5f3b3b21c462694c30a)

Fix bug #8012 (Use getgrset() instead of initgroups() + getgroups() when
getgrouplist() is not defined).