Kernel: Implement multi-watch InodeWatcher :^)

This patch modifies InodeWatcher to switch to a one watcher, multiple
watches architecture.  The following changes have been made:

- The watch_file syscall is removed, and in its place the
  create_iwatcher, iwatcher_add_watch and iwatcher_remove_watch calls
  have been added.
- InodeWatcher now holds multiple WatchDescriptions for each file that
  is being watched.
- The InodeWatcher file descriptor can be read from to receive events on
  all watched files.

Co-authored-by: Gunnar Beutner <gunnar@beutner.name>

Kernel/API/InodeWatcherEvent.h

@@ -1,21 +1,37 @@
 /*
- * Copyright (c) 2020, the SerenityOS developers.
+ * Copyright (c) 2020-2021, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/EnumBits.h>
 #include <AK/Types.h>
 
+#ifdef KERNEL
+#    include <LibC/limits.h>
+#else
+#    include <limits.h>
+#endif
+
 struct [[gnu::packed]] InodeWatcherEvent {
-    enum class Type {
+    enum class Type : u32 {
         Invalid = 0,
-        Modified,
-        ChildAdded,
-        ChildRemoved,
+        MetadataModified = 1 << 0,
+        ContentModified = 1 << 1,
+        Deleted = 1 << 2,
+        ChildCreated = 1 << 3,
+        ChildDeleted = 1 << 4,
     };
 
+    int watch_descriptor { 0 };
     Type type { Type::Invalid };
-    unsigned inode_index { 0 };
+    size_t name_length { 0 };
+    // This is a VLA which is written during the read() from the descriptor.
+    const char name[];
 };
+
+AK_ENUM_BITWISE_OPERATORS(InodeWatcherEvent::Type);
+
+constexpr unsigned MAXIMUM_EVENT_SIZE = sizeof(InodeWatcherEvent) + NAME_MAX + 1;