_XDefaultIOError: Do better at detecting explicit shutdown

Currently, when the X server crashes or a client is disconnected with
XKillClient, you get a somewhat confusing error message from libX11
along the lines of:

XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
      after 98 requests (40 known processed) with 0 events remaining.

What's happening here is the previous recvmsg has thrown EAGAIN, since
the socket is non-blocking. In this case, check whether the socket has
any more data to read, and if not treat it like EPIPE.

Signed-off-by: Adam Jackson <ajax@redhat.com>