Introduce a new sysctl flag, CTLFLAG_SKIP, which will cause

sysctl_sysctl_next() to skip this sysctl.  The sysctl is
still available, but doesn't appear in a "sysctl -a".

This is especially useful when you want to deprecate a sysctl,
and add a warning into it to warn users that they are using
an old interface.  Without this flag, the warning would get
echoed when running "sysctl -a" (which happens at boot).

sys/kern/kern_sysctl.c

@@ -542,6 +542,9 @@ sysctl_sysctl_next_ls(struct sysctl_oid_list *lsp, int *name, u_int namelen,
 		*next = oidp->oid_number;
 		*oidpp = oidp;
 
+		if (oidp->oid_kind & CTLFLAG_SKIP)
+			continue;
+
 		if (!namelen) {
 			if ((oidp->oid_kind & CTLTYPE) != CTLTYPE_NODE) 
 				return 0;

sys/sys/sysctl.h

@@ -86,6 +86,7 @@ struct ctlname {
 #define CTLFLAG_SECURE	0x08000000	/* Permit set only if securelevel<=0 */
 #define CTLFLAG_PRISON	0x04000000	/* Prisoned roots can fiddle */
 #define CTLFLAG_DYN	0x02000000	/* Dynamic oid - can be freed */
+#define CTLFLAG_SKIP	0x01000000	/* Skip this sysctl when listing */
 
 /*
  * USE THIS instead of a hardwired number from the categories below