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/mbrlen.c

@@ -11,7 +11,7 @@
 
 #include "internal.h"
 
-size_t mbrlen(const char *s, size_t n, mbstate_t *st)
+size_t mbrlen(const char *restrict s, size_t n, mbstate_t *restrict st)
 {
 	static unsigned internal;
 	return mbrtowc(0, s, n, st ? st : (mbstate_t *)&internal);