Per request, keep privilege number 20 reserved.
Per request, keep privilege number 20 reserved.

In my commit that moved uname(), setdomainname() and getdomainname() to
COMPAT_FREEBSD4, I also removed PRIV_SETDOMAINNAME, because it was
already protected by userland_sysctl(). We'd better keep the number 20
reserved, to prevent it from being used again.

Requested by:	rwatson

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