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

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.

Previously, there was code to either use select.select or select.poll,
depending on OS.  This changes it to use the selectors module, using
either SelectSelector or PollSelector, but sharing code otherwise.

When the Message object was changed to store an OPT record, the changes
to use_edns() left a lot of cruft.

Reduce some code duplication in make_query().

Define additional return types in name.pyi