Passively checked hosts and freshness management

For passively checked hosts, freshness check is used to launch the check_command if no passive check is received during the freshness_period. 

The host check_command is usually defined as a check_dummy!2!"No passive check received" to make the host/service become DOWN/CRITICAL when no passive check is received. This command should not be launched when the host is not in a check period, so the freshness check should be disabled only in this case.

 The previous PR concerning this behaviour (#1046) still had a bug ...