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. :^)

Kernel/Process.h

@@ -104,6 +104,7 @@ public:
     void die();
     void finalize();
 
+    int sys$watch_file(const char* path, int path_length);
     int sys$dbgputch(u8);
     int sys$dbgputstr(const u8*, int length);
     int sys$dump_backtrace();