EIO error on background tty reads

[Rafael Kitover]

>This isn't a cygwin internals special case knowledge type of thing.  A grep
>of cygwin sources shows that bg_check is called with one of two arguments:
>either SIGTTOU or SIGTTIN is used.  So, your change effectively makes the
>final "goto seEIO" a no-op since it specifically checks for both inputs and
>bypasses the final goto.>
>Sorry, but if this fixes the problem it is a band-aid.
>
>cgfHere's what I understand about this function, fhandler_termios::bg_check