Subscription id is needed for applications which wish to route
messages based on subscription rather than message type. This
field is now available to user applications by the sub_id
field in the message. Subscription based applications may also
need to have access to the message source, so a get source
function has been added.
Change-Id: I796392a6e3899e8f01a3292796e54e7abd56c32f
Signed-off-by: E. Scott Daniels <daniels@research.att.com>