core: Earlier EOS/EOF in core input_filter when socket is closed.

[ylavic]

• server/core_filters.c: Make ap_core_input_filter() return EOS/EOF earlier when the socket is morphed to an empty/immortal brigade and blocking mode is asked. In GETLINE mode this means returning APR_EOF instead of an empty brigade. In READBYTES mode, when greediness finds this case, it means appending the EOS bucket to the returned brigade (otherwise the next call would return APR_EOF without an EOS being ever returned).