Minor casefiddle.c cleanups

Minor casefiddle.c cleanups

* src/casefiddle.c: Redo recent changes to match GNU style,
and prefer C99-style decls within blocks.
(GREEK_CAPITAL_LETTER_SIGMA): Rename from CAPITAL_SIGMA, so that
we are merely using the Unicode name, and make it a constant
rather than a macro.  All uses changed.
(SMALL_SIGMA): Remove; unused.
(GREEK_SMALL_LETTER_FINAL_SIGMA): Rename from SMALL_FINAL_SIGMA,
and make it a constant rather than a macro.  All uses changed.
(do_casify_multibyte_string): Use ‘verify’ rather than an
unportable static_assertion local.