Add dns.edns.register_type().

This allows an application to register a custom EDNS option type.

Changes to blocking model.

Before this change, the synchronous code would check sockets for
readability or writability before doing nonblocking read or write.
This changes them to attempt the read or write first, and then block
if the operation could not complete.

This also removes the no-longer-needed getpeername() call in tcp(),
which was needed to deal with the case where an unconnected socket was
passed in; waiting for writability would block rather than immediately
return an error.  By attempting the write first, we get the error
immediately.

Use the selectors module.