ssh/knownhosts: a parser for the OpenSSH known_hosts file format

Change-Id: I271c90ff3a6d59e2e075c785a6bdb79e4b0849fa
Reviewed-on: https://go-review.googlesource.com/40354
Run-TryBot: Han-Wen Nienhuys <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Adam Langley <[email protected]>

Author: hanwen