gs2_vfd: accept -g to turn on debug output

The --help output and the manpage both advertise that "-g" and "--debug"
turn on debug output, but only "--debug" ever worked, "-g" was never
implemented.

This commit makes both "--debug" and "-g" turn on debug output.

Thanks to Mark Johnsen for the bug report.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
gs2_vfd: add missing -A, -D, and -R command-line args

The manpage and "--help" info both advertised -A, -D, and -R as
synonyms for --accel-seconds, --decel-seconds, and --braking-resistor,
respectively, but the short forms were never added.

This commit fixes that mistake.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>