Validate M and N dimension inputs to sparse(). * sparse.cc (Fsparse): Use xidx_type_value() to extract an octave_idx_type for M and N inputs or throw an error. Remove call to get_dimensions(). Remove FIXME note about unreachable code. Update error messages to be specific about what is wrong with M or N inputs. * mk-sparse-tst.sh: Remove FIXME notes. Update BIST tests for M and N inputs.

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