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

sys/sys/priv.h

@@ -84,6 +84,7 @@
 #define	PRIV_CLOCK_SETTIME	17	/* Can call clock_settime. */
 #define	PRIV_SETTIMEOFDAY	18	/* Can call settimeofday. */
 #define	PRIV_SETHOSTID		19	/* Can call sethostid. */
+#define	_PRIV_SETDOMAINNAME	20	/* Removed. */
 
 /*
  * Audit subsystem privileges.