Convert tinc to use recvmsg and sendmsg for transferring packets

This gives "easy" (hah!) access to the TTL/HOPLIMIT, and
TOS/IPV6_TCLASS fields of the ip/ipv6 headers.

Stopped overloading priority with TOS.

Tested on ipv6 bridging only.

I chickened out when it came to passing ecn through, I think
a remote server should have to agree to pass ecn before allowing
it.

Secondly I added support for the linux timestamping API. I have
long thought that vpn software tended to bottleneck in the
udp recv buffer, and with timestamping on the inbound packets,
we could "see" that, and start taking appropriate action,
like FQ or codeling the flows.

Or... after I recover from this hacking session, sendmmsg
and recvmmesg await.