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/stdio/vsnprintf.c

@@ -10,7 +10,7 @@ static size_t sn_write(FILE *f, const unsigned char *s, size_t l)
 	return l;
 }
 
-int vsnprintf(char *s, size_t n, const char *fmt, va_list ap)
+int vsnprintf(char *restrict s, size_t n, const char *restrict fmt, va_list ap)
 {
 	int r;
 	char b;