Please allow a release exception for nbd_1:2.8.7-4

[Wouter Verhelst <wouter@debian.org>]

Hi,

Subject says it all. Changes are limited to nbd-server's postinst, and
are needed to fix #406963, which is RC.

Thanks,

-- 
Fun will now commence
  -- Seven Of Nine, "Ashes to Ashes", stardate 53679.4