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/fopen.c

@@ -1,6 +1,6 @@
 #include "stdio_impl.h"
 
-FILE *fopen(const char *filename, const char *mode)
+FILE *fopen(const char *restrict filename, const char *restrict mode)
 {
 	FILE *f;
 	int fd;