Use 'struct callout' instead of 'struct callout_handle' to avoid

Use 'struct callout' instead of 'struct callout_handle' to avoid
exhausting the kernel timeout table. Perform the usual gymnastics to
avoid race conditions between node shutdown and timeouts occurring.

Also fix a bug in handling ack delays < PPTP_MIN_ACK_DELAY. Before,
we were ack'ing immediately. Instead, just impose a minimum ack delay
time, like the name of the macro implies.

MFC after:	1 week