Add a missing configuration parameter.
Finishing the "three-equation model" code.

This commit adds the "-clip_shelf_base_salinity" command-line option (so
that one can do "-no_clip_shelf_base_salinity") and the corresponding
configuration flag ocean_three_equation_model_clip_salinity, plus
updates documentation in forcing.tex.

This code should be ready to be ported to 0.6.