Fix ctypes type differences on Darwin.

Makes all twisted.trial tests pass on Snow Leopard.
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 pull request #21 from seanlynch/darwin_ctypes_type_fixes

Fix ctypes type differences on Darwin.
make shared objects too