proposal: x/crypto/ssh: new callbacks to Config and error type

[ThomasHabets]

What version of Go are you using (go version)?

1.9.4 (but it doesn't matter, this isn't in the stdlib yet)

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

Doesn't matter.

What did you do?

I'm trying to get more info from SSH servers, and the API of x/crypto/ssh is not very friendly.

1. I can only list acceptable crypto algorithms, not find out which the remote end support. One reason for wanting this is that I want to check all host keys, but the HostKeyCallback is only called for the negotiated one. The error message includes all algorithms remote end supports, but it's not programmatically extractable.
2. No good way to reject host key and know that's why Dial failed.

What did you expect to see?

1. Config.Remote{Ciphers,KeyExchanges,MACs} callbacks
2. Being able to identify the error returned by ClientConfig.HostKeyCallback so that I know it failed because I wanted it to.

What did you see instead?

API with deficiencies.

Suggested improvements

1. Add Remote{Ciphers,KeyExchanges,MACs} callbacks to type Config
2. Add type HostKeyError, wrapping the error returned from the callback.

[agnivade]

/cc @FiloSottile