[th/lldp-listener-cleanup] simplify NMLldpListener

Closed Thomas Haller requested to merge th/lldp-listener-cleanup into main

Simplify the NMLldpListener API, so that we enable/destroy the NMLldpListener instance to start/stop LLDP. There is no more further state, like running. If it exists, it's running. The goal is to simplify NMDevice, which uses this API.

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