mrp: change registrar state machine to ignore rIn! events.

Previous behavior was for mrpd to emit an error message if an rIn! event was
ever sent to the registrar state machine. This change silently ignores the
rIn! event in the registrar state machine.
MRP: examples: helper: add enums for listener declaration types (avoids magic numbers in the code)
MRP client: fix potential memory leak in error return path of mrpdhelper_to_string()
Merge pull request #28 from audioscience/asi-for-upstream-avb-next

Minor MRP updates