If the subprocess was trying to write to its stdout/stderr, its process
would never actually finish because it was blocked waiting for us to read
it, but we were blocked on waitpid().  Instead, use waitpid(WNOHANG) and
continually read from the subprocess (which should be a blocking operation)
until it exits.

I guess we were causing the kernel to syslog on every single packet on
MacOS.  Oops.

There was a problem where trying to bind .v4 and .v6 listeners would set them
to None if there was nothing to bind (if, say, you weren't binding an IPv6
listener).  Later, the code then would try to call a member function of the
listener.  The member function would not do anything if there was no listener,
but trying to dereference None yielded the broken behavior.

address families.

connections.