MEDIUM: checks: Implement agent check using tcp-check rules

MEDIUM: checks: Implement agent check using tcp-check rules

A shared tcp-check ruleset is now created to support agent checks. The following
sequence is used :

    tcp-check send "%[var(check.agent_string)] log-format
    tcp-check expect custom

The custom function to evaluate the expect rule does the same that it was done
to handle agent response when a custom check was used.