Use freezero(3) for the imsg framework in imsg_free(3) and ibuf_free(3).

Use freezero(3) for the imsg framework in imsg_free(3) and ibuf_free(3).

In our privsep model, imsg is often used to transport sensitive
information between processes.  But a process might free an imsg, and
reuse the memory for a different thing.  iked uses some
explicit_bzero() to clean imsg-buffer but doing it in the library with
the freezero() is less error-prone and also benefits other daemons.

OK deraadt@ jsing@ claudio@