an adaptation of /n/sources/patch/telnet-notkbd from stallion.
the readme is:

This patch corrects a couple of minor problems with telnet when using
the notkbd option (-n).

The first issue was to disable changing raw mode when a pipe was being
used.  This would result in raw mode being enabled on the calling
console.

The second issue was a deadlock caused by the net child being handed a
terminal note.  This would cause the parent to spin waiting for the
child to notify.  This was changed to a waitpid loop such that the
main thread can shutdown gracefully if needed.

Reference: /n/atom/patch/applied2013/telnetnokbd
Date: Wed Dec 11 15:28:17 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/ip/telnet.c	Wed Dec 11 15:27:35 2013
+++ /sys/src/cmd/ip/telnet.c	Wed Dec 11 15:27:35 2013
@@ -164,12 +164,13 @@
 		netpid = pid;
 		notify(notifyf);
 		fromkbd(net);
-		if(notkbd)
-			for(;;)
-				sleep(0);
+		if(notkbd){
+			while(waitpid() != -1)
+				;
+		}else
+			sendnote(netpid, "die");
 		if (svc)
 			remove(svc);
-		sendnote(netpid, "die");
 		exits(0);
 	}
 }
@@ -325,7 +326,7 @@
 void
 consctlcmd(char *s)
 {
-	if(srv != nil)
+	if(srv != nil || notkbd)
 		return;
 	if(debug)
 		fprint(2, "consctl: %s\n", s);