Add sync() syscall and a /bin/sync.

It walks all the live Inode objects and flushes pending metadata changes
wherever needed.

This could be optimized by keeping a separate list of dirty Inodes,
but let's not get ahead of ourselves.

LibC/unistd.cpp

@@ -294,4 +294,9 @@ void _exit(int status)
     assert(false);
 }
 
+void sync()
+{
+    Syscall::invoke(Syscall::SC_sync);
+}
+
 }