MEDIUM: checks: Add supplementary agent checks

MEDIUM: checks: Add supplementary agent checks

Allow an auxiliary agent check to be run independently of the
regular a regular health check. This is enabled by the agent-check
server setting.

The agent-port, which specifies the TCP port to use for the agent's
connections, is required.

The agent-inter, which specifies the interval between agent checks and
timeout of agent checks, is optional. If not set the value for regular
checks is used.

e.g.
server	web1_1 127.0.0.1:80 check agent-port 10000

If either the health or agent check determines that a server is down
then it is marked as being down, otherwise it is marked as being up.

An agent health check performed by opening a TCP socket and reading an
ASCII string. The string should have one of the following forms:

* An ASCII representation of an positive integer percentage.
  e.g. "75%"

  Values in this format will set the weight proportional to the initial
  weight of a server as configured when haproxy starts.

* The string "drain".

  This will cause the weight of a server to be set to 0, and thus it
  will not accept any new connections other than those that are
  accepted via persistence.

* The string "down", optionally followed by a description string.

  Mark the server as down and log the description string as the reason.

* The string "stopped", optionally followed by a description string.

  This currently has the same behaviour as "down".

* The string "fail", optionally followed by a description string.

  This currently has the same behaviour as "down".

Signed-off-by: Simon Horman <horms@verge.net.au>