ssh/terminal: consistent return value for Restore

This patch makes the Restore function return nil
on success to be consistent with other functions
like MakeRaw.

Change-Id: I81e63f568787dd88466a5bb30cb87c4c3be75a5c
Reviewed-on: https://go-review.googlesource.com/34952
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>