MINOR: sink: distinguish between hard and soft close in _sink_forward_io_handler()

MINOR: sink: distinguish between hard and soft close in _sink_forward_io_handler()

Aborting the socket on soft-stop is not the same as aborting it due to
unexpected error. As such, let's leverage the granularity offered by
sedesc flags to better reflect the situation: abort during soft-stop is
handled as a soft close thanks to EOI+EOS flags, while abort due to
unexpected error is handled as hard error thanks to ERROR+EOS flags.

Thanks to this change, hard error will always emit RST packet even if
the NOLINGER option wasn't set on the socket.