Unicode updates and ssh-agent frameworks

• Update to libdot 1.18. Unicode 10.0.0 updates (from Unicode 5).
• Update to hterm 1.72. Misc bugfixes.
• Implement an extensible SSH agent (stub backend).
• Integrate Google Smart Card Connector client.
• Add an agent backend for smart card keys.
• Also set relay page to black background.
• Fail gracefully when loading an unknown profile.
• Fix argstr parsing when connecting via URIs.
• Fix opening a new window from the omnibox.
• Use lib.f.getStack helper in more places.