Agent protocol support

[Roguelazer]

Adds support for using HAProxy 1.7.x (or 1.6.x with a patch I wrote) with the agent protocol and agent-send.

• Adds -A argument which binds a new simple server on a different port. Use the agent-send parameter in your haproxy backends to send requests to this server.
• Adds S argument to disable returning spool (haup / hadown) results on regular healthchecks -- those are only returned on the agent server if you pass -S. That way, deploys and such will put servers in MAINT through the agent, not in DOWN through the regular check

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