Add support for callbacks

Callbacks are the response in the PDM 'trigger->response' model.
Add general support for implementing callbacks and implement
a log to systemd journal using

Add support for callbacks

Callbacks are the response in the PDM 'trigger->response' model.
Add general support for implementing callbacks and implement
a log to systemd journal using that framework.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Change-Id: I8bead5368ee5472a02b47e8bba9e9df3a1f346bc

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