Directly expose the SSH server KEXT, MAC and Cipher algorithms

[belak]

I believe this is one of the last pieces needed for go-gitea/gitea#3896

There may be a better interface for this, but it looks like this was designed so you can change the server config in the middle of running so there's a little extra code to make sure the slice gets copied and not just referenced.

[belak]

Note that I may end up dropping this if we don't need this for gitea.

[shazow]

In x/crypto/ssh, these are buried/bundled under another struct layer of Config: https://godoc.org/golang.org/x/crypto/ssh#Config

Would it be a good idea to do that here as well? Especially since most people won't need them.

Would it make sense to use the original ssh.Config type and pass it along?

[belak]

I did consider exposing the raw SSH config. I suppose that makes more sense since it should only be needed in very specific cases.

Do you have any opinions on copying the config vs just referencing it? I find it a little odd that we create a new server config for every connection.

Thanks for your comments!

[shazow]

I don't have strong feelings either way, just an idea. :)

[progrium]

I could go either way. At the moment at least.

[belak]

I'd like to clean up some of the config handling and make it per call to listen, but that doesn't need to happen right now.