MINOR: tcpcheck: Add support of L7OKC on expect rules error-status argument

MINOR: tcpcheck: Add support of L7OKC on expect rules error-status argument

L7OKC may now be used as an error status for an HTTP/TCP expect rule. Thus
it is for instance possible to write:

    option httpchk GET /isalive
    http-check expect status 200,404
    http-check expect status 200 error-status L7OKC

It is more or less the same than the disable-on-404 option except that if a
DOWN is up again but still replying a 404 will be set to NOLB state. While
it will stay in DOWN state with the disable-on-404 option.