Kernel+LibC: Don't allow a directory to become a subdirectory of itself

If you try to do this (e.g "mv directory directory"), sys$rename() will
now fail with EDIRINTOSELF.

Dr. POSIX says we should return EINVAL for this, but a custom error
code allows us to print a much more helpful error message when this
problem occurs. :^)

Libraries/LibC/errno_numbers.h

@@ -99,4 +99,5 @@
 #define EPROTO 71
 #define ENOTSUP 72
 #define EPFNOSUPPORT 73
-#define EMAXERRNO 74
+#define EDIRINTOSELF 74
+#define EMAXERRNO 75