Introduce raw TCP SPTPS packet transport.

Introduce raw TCP SPTPS packet transport.

Currently, SPTPS packets are transported over TCP metaconnections using
extended REQ_KEY requests, in order for the packets to pass through
tinc-1.0 nodes unaltered. Unfortunately, this method presents two
significant downsides:

 - An already encrypted SPTPS packet is decrypted and then encrypted
   again every time it passes through a node, since it is transported
   over the SPTPS channels of the metaconnections. This
   double-encryption is unnecessary and wastes CPU cycles.

 - More importantly, the only way to transport binary data over
   standard metaconnection messages such as REQ_KEY is to encode it
   in base64, which has a 33% encoding overhead. This wastes 25% of the
   network bandwidth.

This commit introduces a new protocol message, SPTPS_PACKET, which can
be used to transport SPTPS packets over a TCP metaconnection in an
efficient way. The new message is appropriately protected through a
minor protocol version increment, and extended REQ_KEY messages are
still used with nodes that do not support the new message, as well as
for the intial handshake packets, for which efficiency is not a concern.

The way SPTPS_PACKET works is very similar to how the traditional PACKET
message works: after the SPTPS_PACKET message, the raw binary packet is
sent directly over the metaconnection. There is one important
difference, however: in the case of SPTPS_PACKET, the packet is sent
directly over the TCP stream completely bypassing the SPTPS channel of
the metaconnection itself for maximum efficiency. This is secure because
the SPTPS packet that is being sent is already encrypted with an
end-to-end key.