MAJOR: agent: rework the response processing and support additional actions

MAJOR: agent: rework the response processing and support additional actions

We now retrieve a lot of information from a single line of response, which
can be made up of various words delimited by spaces/tabs/commas. We try to
arrange all this and report whatever unusual we detect. The agent now supports :
  - "up", "down", "stopped", "fail" for the operational states
  - "ready", "drain", "maint" for the administrative states
  - any "%" number for the weight
  - an optional reason after a "#" that can be reported on the stats page

The line parser and processor should move to its own function so that
we can reuse the exact same one for http-based agent checks later.