ir_irmp: sigrok PD, make use of IRMP decoder core locking

ir_irmp: sigrok PD, make use of IRMP decoder core locking

Rephrase how the external IRMP library gets loaded, to provide better
diagnostics to users. All decoder instances are equal after the recent
introduction of locking support.

Move the "reset state" call for the IRMP decoder core to the .decode()
method's main loop, where the context manager holds the instance lock.
This allows "parallel" execution of multiple IRMP decoders in the same
sigrok application, assuming that the context manager scope will be
left at some point in time.

This fixes bug #1581 when applications communicate EOF to decoders.

Move some Python object members to local variables. They exclusively
are used within the .decode() method.

Update the copyright for the non-trivial changes.