first of all, POLLERR is supposed to be ignored in `.events` and only
returned in `.revents`.
secondly select()'s exceptfds does not have a 1:1 correspondence with
POLLERR. thankfully, the only caller of this function (in terminal-unix)
never set the POLLERR flag so the errorfds were unused anyways.
so go ahead and remove it entirely instead of pretending we can emulate
something that's not possible.
This implements a poll-compatible interface, backed by select on macOS,
suitable for polling on device files - which are not supported by
macOS's implementation of poll. This is a (long-standing) bug in macOS,
so hopefully we can eventually remove this shim.
