fix check_mk_agent.openbsd multi network interfaces support

[coreidcc]

current behaviour: the script work well if there is only one network interface configured but as soon as more then one network interface exists the reports just garbage.

the cause: seeting IFS to '' will skip any substitution resulting in for loop only executing once (the list to iterate over just consists of a single value)

solution: set IFS to only contain a newline

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