Make :ssh-unknown-hostkey public and support output stream in scp-get

[simkoc]

While using this library I ran into two problems:

1. :ssh-unknown-hostkey was not public in the package and I was (thus) unable to appropriately handle the exception and register the hostkey
2. Sometimes I just want to pass a stream for the data I get with scp-get. However, I had to pass a file-name and this gave me trouble. My usecase was using temporary files with cl-fad. As with-open-temporary-file already opens an fd-stream, passing the pathname of the stream to scp-get would lead to another fd-stream opened on the file which seems to be problematic or at least redundant.

If either use-case is already supported I would be interested to know how, else I hope my code suggestions are useful.