Improve protection against unapproved service deletion.

Improve protection against unapproved service deletion.

If a privilege port was used to register a service, then a privileged
port must be used to unregister it.
This allows system services (statd/lockd/mountd) to safely register
non-privileged ports.
This is good because they don't really need privileged ports, and
using privileged ports when not needed is a waste of scarce recourses.