gfapi: introduce glfs_readdir() and glfs_readdirplus() APIs

Change-Id: I6b233bf647585675f233898351bf593f251716cc
BUG: 839950
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/6201
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>