LU-2107 util: Enhance l_getidenty to leverage 'nscd' cacheing

Replace getgrent() use with getgrouplist() in l_getidentity,
which would allow it to leverage the 'nscd' cache.

Xyratex-bug-id: MRP-643
Signed-off-by: Sergii Glushchenko <sergii_glushchenko@xyratex.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Iurii Golovach <iurii_golovach@xyratex.com>
Change-Id: I593abaeefe02cdb2d4f8761124bdd48477a7a22a
Reviewed-on: http://review.whamcloud.com/4223
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Iurii Golovach <iurii.golovach@gmail.com>