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/string/wcstok.c

@@ -1,6 +1,6 @@
 #include <wchar.h>
 
-wchar_t *wcstok(wchar_t *s, const wchar_t *sep, wchar_t **p)
+wchar_t *wcstok(wchar_t *restrict s, const wchar_t *restrict sep, wchar_t **restrict p)
 {
 	if (!s && !(s = *p)) return NULL;
 	s += wcsspn(s, sep);