fixed g_io_channel_seek_position() so that G_SEEK_CUR works for UTF-8

fixed g_io_channel_seek_position() so that G_SEEK_CUR works for UTF-8

        * glib/giochannel.c: fixed g_io_channel_seek_position() so that
        G_SEEK_CUR works for UTF-8 channel encoding, and unallocated
        converters are not flushed after the seek; fixed
        g_io_channel_get_buffer_condition() so that G_IO_IN is only
        set if the read buffer contains at least one full character