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. :^)

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -522,6 +522,11 @@ KResult VFS::rename(StringView old_path, StringView new_path, Custody& base)
     if (&old_parent_inode.fs() != &new_parent_inode.fs())
         return KResult(-EXDEV);
 
+    for (auto* new_ancestor = new_parent_custody.ptr(); new_ancestor; new_ancestor = new_ancestor->parent()) {
+        if (&old_inode == &new_ancestor->inode())
+            return KResult(-EDIRINTOSELF);
+    }
+
     auto current_process = Process::current();
     if (!new_parent_inode.metadata().may_write(*current_process))
         return KResult(-EACCES);

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

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 :^)",
 };