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. :^)

Userland/mon.cpp

@@ -0,0 +1,29 @@
+#include <AK/LogStream.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    const char* path = argc > 1 ? argv[1] : ".";
+    int watch_fd = watch_file(path, strlen(path));
+    if (watch_fd < 0) {
+        perror("Unable to watch");
+        return 1;
+    }
+    for (;;) {
+        char buffer[256];
+        int rc = read(watch_fd, buffer, sizeof(buffer));
+        if (rc < 0) {
+            perror("read");
+            return 1;
+        }
+        if (rc == 0) {
+            printf("End-of-file.\n");
+            return 0;
+        }
+        printf("Something changed about '%s'\n", path);
+    }
+    return 0;
+}