Holland and Jenkins melt parameterisation issues

[claireyung]

Hello,

I found some issues in the MOM6 ice shelf melt parameterisation module (a missing von Karman constant in the definition of ZETA_N, buoyancy iteration loop that doesn't progress and salt iteration overriding the false position method) which were explained and addressed in PR NOAA-GFDL/MOM6#395. I believe the same bugs are present in the iceberg melt code:

Missing von Karman constant in ZETA_N: https://github.com/NOAA-GFDL/icebergs/blob/dev/gfdl/src/icebergs.F90#L3514
Buoyancy iteration loop doesn't update: https://github.com/NOAA-GFDL/icebergs/blob/dev/gfdl/src/icebergs.F90#L3697
Salt iteration overrides false position method a line earlier: https://github.com/NOAA-GFDL/icebergs/blob/dev/gfdl/src/icebergs.F90#L3755