Fix incorrect teredo MTU calculation

libUTP overestimated maximum payload size for UDP packets, which
resulted in fragmented datagrams on IPv6 links with MTU 1280 (e.g.
teredo, 6to4, etc).
Merge pull request #41 from yumkam/master

Fix incorrect teredo MTU calculation
Merge https://github.com/bittorrent/libutp