Update frz_onset and mlt_onset restart and history capability

[apcraig]

frz_onset and mlt_onset do not restart properly and they are unable to restart bit-for-bit in history files. This is a long standing issue that we should correct. I believe the main solution is to add these fields to the restart file, but additional assessments may be needed.

[dabail10]

Actually, frz_onset is on the restart, but mlt_onset is not. Not sure why it was done this way. We should definitely add it.

[dabail10]

I see now. frz_onset is used for restarting the first-year ice tracer (tr_FY). I guess we should add frz_onset and mlt_onset normally. Separate PR.