MINOR: checks: Set by default expect rule status to UNKNOWN during parsing

MINOR: checks: Set by default expect rule status to UNKNOWN during parsing

The status (ok, error and timeout) of an TCP or HTTP expect rule are set to
HCHK_STATUS_UNKNOWN by default, when not specified, during the configuration
parsing. This does not change the default status used for a terminal expect rule
(ok=L7OK, err=L7RSP and tout=L7TOUT). But this way, it is possible to know if a
specific status was forced by config or not.