+*Fix 3-equation ice-ocean flux iteration (#972)

Fix the 3-equation iteration for the buoyancy flux between the ocean and an
overlying ice-shelf when ICE_SHELF_BUOYANCY_FLUX_ITT_BUGFIX is true and
SHELF_3EQ_GAMMA it false.  This code now uses proper bounding of the
self-consistent solution, avoiding further amplifying the fluxes in the cases
when the differences between the diffusivities of heat and salt to make the
buoyancy flux destabilizing for finite turbulent mixing.  Both the
false-position iterations and the (appropriately chosen) Newton's method
iterations have been extensively examined and determined to be working correctly
via print statements that have subsequently been removed for efficiency.

  Previously, the code to determine the 3-equation solution for the buoyancy
flux between the ocean and an ice shelf had been skipping iteration altogether
or doing un-bounded Newton's method iterations with a sign error in part of the
derivative, including taking the square root of negative numbers, leading to the
issue described at #945.  That issue has
now been corrected and can be closed once this commit has been merged into
the dev/gfdl branch of MOM6.

  This commit also changes the names of the runtime parameters to correct the
ice shelf flux iteration bugs from ICE_SHELF_BUOYANCY_FLUX_ITT_BUG and
ICE_SHELF_SALT_FLUX_ITT_BUG to ICE_SHELF_BUOYANCY_FLUX_ITT_BUGFIX and
ICE_SHELF_SALT_FLUX_ITT_BUGFIX to avoid confusion with other ..._BUG parameters
where `true` is to turn the bugs on, whereas here `true` fixes them.  The old
names are retained via `old_name` arguments to the `get_param()` calls, so no
existing configurations will be disrupted by these changes.

  Additionally, an expression to determine a scaling factor to limit ice-shelf
bottom slopes in `calc_shelf_driving_stress()` was refactored to avoid the
possibility of division by zero.

  This commit will change (and correct) answers for cases with
ICE_SHELF_BUOYANCY_FLUX_ITT_BUGFIX set to true, but as these would often fail
with a NaN from taking the square root of a negative value, it is very unlikely
that any such configurations are actively being used, and there seems little
point in retaining the previous answers.  No answers are changed in cases that
do not use an active ice shelf.

Co-authored-by: Alistair Adcroft <adcroft@users.noreply.github.com>

Author: Hallberg-NOAA