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/dirent/readdir_r.c

@@ -5,7 +5,7 @@
 #include "__dirent.h"
 #include "libc.h"
 
-int readdir_r(DIR *dir, struct dirent *buf, struct dirent **result)
+int readdir_r(DIR *restrict dir, struct dirent *restrict buf, struct dirent **restrict result)
 {
 	struct dirent *de;
 	int errno_save = errno;