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.

The existing receive_udp() methods are only usable for receiving
responses, as they require an expected destination and check that the
message is from that destination.

This change makes the expected destination (and hence the check)
optional, and returns the address that the message was received from (in
the sync case, this is only done if no destination is provided, for
backwards compatibility).

New tests are added, which required adding generic getsockname() support
to the async backends.