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/FileDescription.h

@@ -77,6 +77,10 @@ public:
     const TTY* tty() const;
     TTY* tty();
 
+    bool is_inode_watcher() const;
+    const InodeWatcher* inode_watcher() const;
+    InodeWatcher* inode_watcher();
+
     bool is_master_pty() const;
     const MasterPTY* master_pty() const;
     MasterPTY* master_pty();