Modifying RadarParmDecode to set ifmode to zero by default

[egthomas]

This pull request addresses an inconsistency identified by Sessai, where currently the RadarParmDecode function sets the ifmode field to -1 if the field is not present, eg for files written by QNX4-era (or earlier) radars. This pull request modifies the behavior such that the ifmode field is always set to zero (0) by default (which matches our documentation).

To test, you can try something like this:

make_fit -fitacf2 20230129.0616.03.sys.rawacf > 20230129.0616.03.sys.fitacf
dmapdump 20230129.0616.03.sys.fitacf > 20230129.0616.03.sys.fitacf.dump
grep -r "ifmode" 20230129.0616.03.sys.fitacf.dump | tail -1

On this branch, the ifmode field in the radar parameter block should be 0, while on develop it will be -1. No other functionality has changed.