Extra inline helpers seat_{stdout,stderr}_pl.

These take a ptrlen in place of separate buffer and length arguments.
Switched over to them in lots of places.

sshcommon.c

@@ -804,7 +804,7 @@ void ssh_ppl_user_output_string_and_free(PacketProtocolLayer *ppl, char *text)
     /* Messages sent via this function are from the SSH layer, not
      * from the server-side process, so they always have the stderr
      * flag set. */
-    seat_stderr(ppl->seat, text, strlen(text));
+    seat_stderr_pl(ppl->seat, ptrlen_from_asciz(text));
     sfree(text);
 }