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/CMakeLists.txt

@@ -198,7 +198,7 @@ set(KERNEL_SOURCES
     Syscalls/unveil.cpp
     Syscalls/utime.cpp
     Syscalls/waitid.cpp
-    Syscalls/watch_file.cpp
+    Syscalls/inode_watcher.cpp
     Syscalls/write.cpp
     TTY/MasterPTY.cpp
     TTY/PTYMultiplexer.cpp