AT command APDU has trouble communicating with Quectel modem

[ecm0708]

There's a problem using the AT command APDU with quectel modems. It seems to be related to the method of writing to the serial device.

@steely-glint has a fork which solves the problem. In driver/apdu/at.c, it replaces fprintf(fuart...) with possibly smaller sized write(uart_fd...). After merging this fork with the current upstream and reducing the maximum segment size ctx->es10x_mss, it works well on a Quectel modem. I haven't tested on another modem. Hopefully, this method of communicating through the serial device would also be compatible with other modems. (Or maybe an environment variable could be used to select the method.) It looks like the fork on its own does not work since it doesn't have the ability to reduce ctx->ex10x_mss from upstream.

Would it be feasible to merge this upstream? If needed, I could provide a patch or PR with the merge conflicts resolved.
main...pipe:lpac-quectel:main#diff-d85497acc8400d3fa4f69b00a762a91dc21f1ef20bdacc9e37cf46218c529287R99

[CoelacanthusHex]

In driver/apdu/at.c, it replaces fprintf(fuart...) with possibly smaller sized write(uart_fd...).

It may be caused by buffered io. Invoking setbuf(uart_fd, NULL) after fopen to disable buffered I/O should fix the issue.