This fixes one spec on Darwin:

Signal.list redefines CLD with CHLD if defined FAILED
Expected nil == 20
to be truthy but was false

This expands our exception translation to work for more than just
the IOException family of types, first including the
NotYetConnectedException that does not live within that hierarchy.

In addition, it makes this translation method explicitly from
Throwable to Throwable with a user-provided fallback, and adds a
method for doing this translation or throwing the original error
in one shot.

This change was motivated by issues in BasicSocket where the
NotYetConnectedException was bubbling out as-is, rather than being
translated into an ENOTCONN. A subsequent change will modify
RubyBasicSocket to use this new logic.

Without this change, we were raising the original NotYetConnected
exception instead of a more appropriate ENOTCONN.

When the socket is a datagram socket, implement these using
DatagramChannel.send and receive. Otherwise fall back on the
previous logic that does writes and reads.

This fixes all failures in BasicSocket's read_nonblock specs.

This fixes at least one spec failure on Darwin, and may fix
others.

MacOS appears to provide a hostname for the broadcast address, so
when we attempt to reverse the address we get the name instead of
the address. We shouldn't reverse the name, but there doesn't
appear to be another way to get the hostname from Java's
InetAddress.

* On Linux, read_nonblock's use of recv(... MSG_DONTWAIT) does not
  leave the socket non-blocking, so we emulate that with Java APIs
  here.
* Refactor handling of EAGAIN result to share code with
  recv_nonblock.

This moves some hanging specs to a new "hangs" tag, to make that
exclusion clearer and filterable when updating "fails" tags.