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/fputs.c

@@ -1,6 +1,6 @@
 #include "stdio_impl.h"
 
-int fputs(const char *s, FILE *f)
+int fputs(const char *restrict s, FILE *restrict f)
 {
 	size_t l = strlen(s);
 	if (!l) return 0;