use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

src/multibyte/wcstombs.c

@@ -11,7 +11,8 @@
 
 #include "internal.h"
 
-size_t wcstombs(char *s, const wchar_t *ws, size_t n)
+size_t wcstombs(char *restrict s, const wchar_t *restrict ws, size_t n)
 {
-	return wcsrtombs(s, &ws, n, 0);
+	const wchar_t * x = ws;
+	return wcsrtombs(s, &x, n, 0);
 }