bug#55360: bug#58375: Installer does not show what is being downloaded

[Mathieu Othacehe]

Hey,

Thanks for having a look!

> I haven’t actually tested the patch but it LGTM.  One thing to check is
> whether ‘terminal-window-size’ returns something sensible for the
> pseudo-terminal; it could be that we need an extra ioctl so the
> pseudo-terminal has the same size as the actual terminal.

Well it returns 0 for all fields, but I tested on several screen sizes
and everything seems fine so I went ahead.

While testing I noticed two new issues though:

1. When the disk is GPT partitionned there is no confirmation page in
  "run-label-page". Something I missed in #57232.

2. When there is an exception in run-external-command-with-handler/tty
    for instance, the backtrace page is displayed in the PTY and the
    keyboard shortcuts do not work anymore.

I'll address point 1 shortly but could use some advice for point 2.

Thanks,

Mathieu