Accept input file name on candump2analyzer command line.

If an argument is given to candump2analyzer, try to use it as the
input file name instead of standard input.  If that fails, return
error exit code 1.
Do not rely on fixed CANDUMP_DATA_START.

Just in case the CAN ID or size fields have a non-standard length, try
to find the data start point by looking for the first non-space
character after the closing bracket.
Increase buffer size for Fast-Packet data.

Set MAX_DATA_BYTES to 223, which is the maximum length of a full
Fast-Packet payload package.  Also check the buffer size to avoid
overflowing.
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.
Fix message lengths for Airmar proprietary PGNs.

According to the quoted manuals, the PGN 126720 variants for Airmar
are all marked as "Single Frame: No".  Where the message lengths are
not greater than 8 bytes, add a comment to warn that the parser might
fail because it expects a single-frame format.

Use RES_NOTUSED for Reserved fields to suppress useless output.

Add a 126720-44 variant, as found in the DST200 User Manual document.
Extend the lookup for the PGN 126720-46 Transmission Interval field (3
means data not available).
Fix PGN 127493 Transmission Parameters, Dynamic.

Based on own research and the official PGN and field list at
http://www.nmea.org/Assets/july%202010%20nmea2000_v1-301_app_b_pgn_field_list.pdf

This PGN is transmitted with DLC=8 bytes and contains another Reserved
Bits field at the end. The Engine Instance is actually 8 bits wide.
The Transmission Gear field is padded with 6 bits to the next byte
boundary for the following 16-bit values.
Merge pull request #22 from acolomb/select-fixed-fields

Allow fall-back from PGN descriptions with fixed fields to more generic ones
Merge pull request #23 from acolomb/transmission-parameters

Fix PGN 127493 Transmission Parameters, Dynamic.
Merge pull request #24 from acolomb/airmar-updates

Fix message lengths for Airmar proprietary PGNs.
Merge pull request #25 from acolomb/candump-improvements

Extend functionality for candump2analyzer tool