Fix alternative PGN description selection logic.

Match a PGN description from the list if either it has no fixed fields
or all fixed fields matched the message.  This gives descriptions with
fixed fields priority but allows a fallback, as long as the
descriptions are ordered with the most specific one first.
Reorder PGN 126720 descriptions.

Move the most generic description last.  The order is now:

1. Airmar-specific with fixed Proprietary ID
2. Airmar-specific without Proprietary ID
3. Generic Manufacturer Proprietary

Also fix the message lengths for these two entries.  Skip the Reserved
fields using RES_NOTUSED.
Add missing generic fields for PGN 61184.

The fields 1 to 3 in this PGN should have a constant meaning, just
like with PGN 126720, its Multi-Frame equivalent.
Merge pull request #22 from acolomb/select-fixed-fields

Allow fall-back from PGN descriptions with fixed fields to more generic ones