network plugin: Copy received value-lists before dispatching them.

network plugin: Copy received value-lists before dispatching them.

A network packet may contain some parts (e.g. hostname) only once. When
parsing the packet, the network plugin incrementally builds up the value list
and dispatches it after parsing a value. This may happen several times without
changing other parts (like the hostname).

However, using the filter-chains, those "static" parts may be overwritten /
modified while dispatching the values. Thus, they cannot reliably be reused
after calling plugin_dispatch_values(). Copying the value list to a temporary
list fixes this.