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/string.cpp

@@ -364,6 +364,7 @@ const char* const sys_errlist[] = {
     "Protocol error",
     "Not supported",
     "Protocol family not supported",
+    "Cannot make directory a subdirectory of itself",
     "The highest errno +1 :^)",
 };