ssh/terminal: implement ReadPassword and IsTerminal
ssh/terminal: implement ReadPassword and IsTerminal

Fixes golang/go#13085.

Change-Id: I2fcdd60e5e8db032d6fa3ce76198bdc7a63f3cf6
Reviewed-on: https://go-review.googlesource.com/16722
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/crypto,ScriptRock/crypto,golang/crypto,ScriptRock/crypto,golang/crypto