ssh: add IUTF8 constant from RFC 8160

Change-Id: I65ac40d86c4f0ee5552cd2e167ac91a735f6c56e
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/392134
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Maisem Ali <maisem@tailscale.com>
diff --git a/ssh/session.go b/ssh/session.go
index d3321f6..eca31a2 100644
--- a/ssh/session.go
+++ b/ssh/session.go
@@ -85,6 +85,7 @@
 	IXANY         = 39
 	IXOFF         = 40
 	IMAXBEL       = 41
+	IUTF8         = 42 // RFC 8160
 	ISIG          = 50
 	ICANON        = 51
 	XCASE         = 52