* feat: enable fwmark (SO_MARK) for outgoing sockets

* fix: make fwmark linux-specific functionality

* fix: minor improvements over handling fwmark

* Use `transport.PacketListener` as interface.

* Take the `syscall.RawConn` as input to `SetFwdmark()`.

* Some cleanup.

* Fix copyright dates for new files.

* Fix the error types.

* Revert changes to integration test.

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Co-authored-by: sbruens <[email protected]>