Correctly sequence RPC cleanup.

Removal of service registrations must be performed prior to
svc_shutdown.  To ensure that, do both in the shutdown thread,
in the correct order.

Take the unrelated but necessary fix for SVC_DESTROY shutdown
fault by Jeremey.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>