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/Syscall.h

@@ -118,7 +118,8 @@ struct timeval;
     __ENUMERATE_SYSCALL(reboot)                 \
     __ENUMERATE_SYSCALL(dump_backtrace)         \
     __ENUMERATE_SYSCALL(dbgputch)               \
-    __ENUMERATE_SYSCALL(dbgputstr)
+    __ENUMERATE_SYSCALL(dbgputstr)              \
+    __ENUMERATE_SYSCALL(watch_file)
 
 namespace Syscall {