Use PACKET instead of SPTPS when talking to neighbors over TCP

This pull request makes tinc uses the `PACKET` metaconnection message when sending packets to a *neighbor* (as opposed to a far-away node) over TCP, even if SPTPS is enabled. This is more efficient than doing SPTPS over SPTPS over TCP, and is just as secure since the metaconnection itself uses SPTPS, and since it's the only link, we still get end-to-end security.

This pull request also stops tinc from spontaneously establishing a SPTPS link with all its neighbors if there are no packets flowing. If using SPTPS this results in the first packet to a neighbor getting lost, but that's not the case if using PACKET.