Remove useless nonneg hypotheses (LInt_SFp_{correct,ext,ext_ex}).

Rename LInt_SFp -> LInt_SF,
       *SFp_* -> *SF_* (when there is no nonneg hypothesis),
       SF_* -> SFplus_* (when there is a nonneg hypothesis),
       Lint_SFp_eq_other_list -> Lint_SFp_skip_0.