gfapi: add new API glfs_set_xlator_option()

This is the equivalent of specifying --xlator-option command line
argument to glusterfsd.

Change-Id: I1171dd518a85db4bd456fa0790f450e889a956cd
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4927
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>