Werk #11719: Levels for packet rates for network interface checks

Previously, the error, multicast and broadcast WARN/CRIT levels specified within the ruleset "Network interfaces and switch ports" were not applied correctly. I.e., all levels were interpreted as percentual levels, regardless of whether they had been specified as absolute or percentual levels. This behavior is fixed with this werk.

Additionally, the forms for error, multicast and broadcast levels now yield a unified structure, that allows the specification of independent levels for the in and out directions.

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