Simplify type coercion in condition evaluation

Simplify type coercion in condition evaluation

Change nstr_mkselval() to generate values with promoted types only,
and replace nstr_coerce_val() by new and simpler nstr_optype() in
nstr_comp().

Replace the only remaining use of nstr_coerce_val() in surv() by
nstr_promote(), and remove nstr_coerce_val().

This loses one half of the unimplemented sketch of coercions to
NSC_STRING.  Drop the other half from nstr_exec_val().