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/FileSystem/File.h

@@ -109,6 +109,7 @@ public:
     virtual bool is_block_device() const { return false; }
     virtual bool is_character_device() const { return false; }
     virtual bool is_socket() const { return false; }
+    virtual bool is_inode_watcher() const { return false; }
 
     virtual FileBlockCondition& block_condition() { return m_block_condition; }