bug#54945: installer should have a no-graphics boot option

[raingloom]

Disclaimer: the last time I used an ISO for installing Guix was before
there was a graphical installer, so I'm not sure what it's expected to
look like now.

Anyways, I tried booting the latest release on a netbook, it failed
around/after KMS. Kernel seemed to lock up. No panic, no usable visible
log, not responding to Ctrl-Alt-Del, had to reboot with Sysrq-Alt-b.
I added nomodeset to the kernel command line in GRUB and managed to
boot it that way, although that still didn't immediately result in a
usable terminal, vtty1 only had a non-blinking cursor if I recall
correctly. Had to switch to vtty2 or above to do anything useful.

So, most users can't be expected to know about nomodeset, so there
should be a clearly labeled "safe graphics" mode, just like how some
other distros do it.

There have been other issues and discussions about generating multiple
configs from a single reconfigure, this would be an important and
practical use case for that.

IMHO the simplest thing is to just let config files return a list of
operating-system records and make that into a menu in GRUB. This would
also cover the use case of wanting a boot time choice in kernels.