Werk #8570: Make PROCESS_HOST_CHECK_RESULT's behavior more Nagios-like

The Nagios documentation for the external command PROCESS_HOST_CHECK_RESULT explicitly states that the return code in the commandline is already the host state, not the result code of a check result (which was the core's previous interpretation). We follow Nagios now more closely and map the return code in the commandline as follows:

0 => UP 1 => DOWN/UNREACHABLE (previously this meant UP, too) 2 => DOWN/UNREACHABLE

We still deviate a bit from Nagios, because the actual decision if a host is DOWN or UNREACHABLE is done by the core and can't be overridden from the outside.

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