MAJOR: checks: Implement HTTP check using tcp-check rules

MAJOR: checks: Implement HTTP check using tcp-check rules

HTTP health-checks are now internally based on tcp-checks. Of course all the
configuration parsing of the "http-check" keyword and the httpchk option has
been rewritten. But the main changes is that now, as for tcp-check ruleset, it
is possible to perform several send/expect sequences into the same
health-checks. Thus the connect rule is now also available from HTTP checks, jst
like set-var, unset-var and comment rules.

Because the request defined by the "option httpchk" line is used for the first
request only, it is now possible to set the method, the uri and the version on a
"http-check send" line.