jaildesc: remove desc from the sysctl parameter list

Like lastjid, desc should count as a pseudo-parameter.  The difference
lies entirely in the security.jail.param sysctls, which list all of
the jail parameters.  Since desc opens and returns a file descriptor,
it has no place in such uses as "jls all."  Like lastjid, it's still
recognized by the kernel if passed in/out, and hard-coded into libjail
so it can be recognized there.

MFC after:	3 days

lib/libjail/jail.c

@@ -920,13 +920,21 @@ jailparam_type(struct jailparam *jp)
 	} desc;
 	int mib[CTL_MAXNAME];
 
-	/* The "lastjid" parameter isn't real. */
+	/*
+	 * Some pseudo-parameters don't show up in the sysctl
+	 * parameter list.
+	 */
 	name = jp->jp_name;
 	if (!strcmp(name, "lastjid")) {
 		jp->jp_valuelen = sizeof(int);
 		jp->jp_ctltype = CTLTYPE_INT | CTLFLAG_WR;
 		return (0);
 	}
+	if (!strcmp(name, "desc")) {
+		jp->jp_valuelen = sizeof(int);
+		jp->jp_ctltype = CTLTYPE_INT | CTLFLAG_RW;
+		return (0);
+	}
 
 	/* Find the sysctl that describes the parameter. */
 	mib[0] = 0;