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/dprintf.c

@@ -1,7 +1,7 @@
 #include <stdio.h>
 #include <stdarg.h>
 
-int dprintf(int fd, const char *fmt, ...)
+int dprintf(int fd, const char *restrict fmt, ...)
 {
 	int ret;
 	va_list ap;