mount.glusterfs: Introduce mem-accounting as an option

option mem-accounting enables memory accounting for the client
process.

re-factored to keep options with values and options without values
in different sections of mount.glusterfs.

Change-Id: I54ebc31a1eae6d7a5ce7b0255cd7df74d37d46c1
BUG: 834465
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/4524
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>