Deal with resizing terminal (xterm.js) and updating stty rows/cols in Linux

We need a solution for resizing our serial (`ttyS0`) in Linux to match whatever dimensions xterm.js is occupying in the client. The serial console doesn't automatically deal with resizing like a network `tty` does. See my comments in https://github.com/copy/v86/issues/172#issuecomment-396976546.

We probably need to open a second channel (another serial link?) and send updated size info when xterm changes size, telling Linux to alter the rows and cols for our tty.

I'm sure this is a solved problem in the Linux ecosystem somewhere, but we need to find and implement it.