Rewrite special file handling on Windows

This change gets GNU grep working. What caused it to not work, is it
wouldn't write to an output file descriptor when its dev/ino equaled
/dev/null's. So now we invent special dev/ino values for these files

libc/calls/struct/fd.internal.h

@@ -11,6 +11,7 @@ COSMOPOLITAN_C_START_
 #define kFdZip      6
 #define kFdEpoll    7
 #define kFdReserved 8
+#define kFdDevNull  9
 
 struct Fd {
   char kind;