Use libgssh for ssh

The "sshpass" program is a useful crutch for some situations, but
ultimately we're going to need a lot more interactive, fine grained
control over ssh.  For example, being able to choose different
authentication types, inspecting host keys, etc.

Additionally, libgssh makes it easy to use the powerful ssh "channel"
feature, which would allow us to do things like spawn a pty+shell on a
separate subchannel, without reauthenticating.