Kernel: Add a mechanism for listening for changes to an inode.

The syscall is quite simple:

    int watch_file(const char* path, int path_length);

It returns a file descriptor referring to a "InodeWatcher" object in the
kernel. It becomes readable whenever something changes about the inode.

Currently this is implemented by hooking the "metadata dirty bit" in
Inode which isn't perfect, but it's a start. :^)

Libraries/LibC/fcntl.h

@@ -54,6 +54,7 @@ __BEGIN_DECLS
 #define S_IRWXO (S_IRWXG >> 3)
 
 int fcntl(int fd, int cmd, ...);
+int watch_file(const char* path, int path_length);
 
 #define F_RDLCK 0
 #define F_WRLCK 1