Try to use UDP to relay SPTPS packets received over TCP.

Currently, when tinc receives a SPTPS packet over TCP via the REQ_KEY
encapsulation mechanism, it forwards it like any other TCP request. This
is inefficient, because even though we received the packet over TCP,
we might have an UDP link with the next hop, which means the packet
could be sent over UDP.

This commit removes that limitation by making sure SPTPS data packets
received through REQ_KEY requests are not forwarded as-is but passed
to send_sptps_data() instead, thereby using the same code path as if
the packet was received over UDP.