x/crypto/ssh/terminal: create stubs for plan9 methods

To facilitate testing of methods in other GOOSs we need plan9 to
be able to build and run the test without a errors due to
undefined methods.

Fixes golang/go#15195

Change-Id: Ida334676f92db6fb4652af3e3a9f6bc13a96052c
Reviewed-on: https://go-review.googlesource.com/21711
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>