add all missing wchar functions except floating point parsers

these are mostly untested and adapted directly from corresponding byte
string functions and similar.

src/string/wcsncasecmp_l.c

@@ -0,0 +1,6 @@
+#include <wchar.h>
+
+int wcsncasecmp_l(const wchar_t *l, const wchar_t *r, size_t n, locale_t locale)
+{
+	return wcsncasecmp(l, r, n);
+}