MEDIUM: checks: Add a list of vars to set before executing a tpc-check ruleset

MEDIUM: checks: Add a list of vars to set before executing a tpc-check ruleset

A list of variables is now associated to each tcp-check ruleset. It is more a
less a list of set-var expressions. This list may be filled during the
configuration parsing. The listed variables will then be set during each
execution of the tcp-check healthcheck, at the begining, before execution of the
the first tcp-check rule.

This patch is mandatory to convert all protocol checks to tcp-checks. It is a
way to customize shared tcp-check rulesets.