libgfapi: Added Handle-based ops to get/set/remove extended attributes in the libgfapi.

Cherry picked from commit Change-Id: I1a8e666018d7b93e0bba2d9882935681da909980
and Change-Id: I62f63da37edf722d6d79c75f72ee7403e93e4936

> BUG: 1089414
> Signed-off-by: Soumya Koduri <skoduri@redhat.com>
> Reviewed-on: http://review.gluster.org/7308
> Reviewed-by: Vijay Bellur <vbellur@redhat.com>
> Tested-by: Vijay Bellur <vbellur@redhat.com>

This patch differs a bit from its earlier master patch -
* 'DECODE_SYNCOP_ERR' macro is not defined in this code branch.
So lines where this macro is used are skipped.
* 'syncop_removexattr(..)' definition differs from the one in the master branch.

BUG: 1099878
Change-Id: I1e9cce4efeec038b9736065d39887c35752caead
Signed-off-by: Soumya Koduri <skoduri@redhat.com>
Reviewed-on: http://review.gluster.org/7825
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>