gfapi: implement readdirplus_r support

This call is used by Samba VFS.

Change-Id: Ib371502ad5a71b3b0e993f6b30e3dfb1f839c020
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5154
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>