route(8): make it possible to manually delete pinned route

Reported by:	Andreas Longwitz <longwitz@incore.de>
Approved by:	avg (mentor)
MFC after:	1 week

sbin/route/route.c

@@ -1519,8 +1519,10 @@ rtmsg(int cmd, int flags, int fib)
 			so[RTAX_IFP].ss_len = sizeof(struct sockaddr_dl);
 			rtm_addrs |= RTA_IFP;
 		}
-	} else
+	} else {
 		cmd = RTM_DELETE;
+		flags |= RTF_PINNED;
+	}
 #define rtm m_rtmsg.m_rtm
 	rtm.rtm_type = cmd;
 	rtm.rtm_flags = flags;