Add host id in data header

Fjen Undso requested to merge hostid into master

Sometimes local DNS resolution can be problematic. An alternative is to send broadcast packages. But that cannot be done if one wants to address one specific node.

This uses one byte of the unused header space to insert a host id. A node only accepts packets, which contain an ID that matches the number in its hostname (like 42 in "ESP42"). A 0 value can be used to trigger the old behaviour, that means all nodes accept it.

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