0.10 regression: Spins in CPU loop with an ssh-dss key in known_hosts

@cryptomilk reported that cockpit's unit tests (in particular, test-sshbridge) fail/time out when running against 0.10. cockpit-ssh uses 100% CPU in an endless loop. When running with G_MESSAGES_DEBUG=all, the cockpit-ssh process spits out and endless stream of

ssh_known_host_sigs_from_hostkey_type:  The given type 1 is not a base private key type or is unsupported

This is related to having a ssh-dss key in the known_hosts data/file.

DSA has been deprecated for a while now, so I am updating Cockpit's tests to not use DSA any more: https://github.com/cockpit-project/cockpit/pull/17710 With that PR, tests are happy against libssh 0.10, but of course this is still a regression that needs to be fixed.