Add a "prompt_login" helper function

[faho]

Description

This prints a description of the "host". Currently that's

(chroot:debianchroot) $USER@$hostname

with the chroot part when needed. That means instead of having one "debian chroot" prompt, we support that out of the box.

This also switches the default and terlar prompts to use it, the other
prompts have slightly different coloring or logic here. It would be possible to allow that by adding options to prompt_host.

Part of #7884.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

Questions:

• Is this wanted at all? Like I said I think having debian chroots off in a separate prompt just results in nobody using it because they would have to change the prompt in the chroot (which means allowing for fish_config to work there!), so the alternative seems to be not supporting that at all
• What about the name? prompt_host sounds like it might be prompt_hostname, but prompt_chroot_user_at_host is a bit wordy. prompt_machine perhaps?
• Support for things other than the debian chroot thing - do other systems have something similar? How about virtual machines? We already color the host differently if it's ssh.

[krobelus]

(#6398 had a more comprehensive solution, using systemd-detect-virt and Debian's ischroot, at the cost of adding forks at startup)

Not a strong opinion but I'd be fine with dropping the debian chroot prompt. Or replace it with the generic systemd-detect-virt --chroot. (We don't care too much about containers/virtualization, because they hopefully have a different hostname from the host, but the chroot one can be nice.) Though I don't know how often this is useful in practise, is chroot still commonly used (over containers)?.

Distribution integration is a bit lacking in general; I wouldn't be surprised if $PATH was not set up correctly when using fish as login shell on a Debian (chroot) system. That would be nice to have fixed but it's not really within our scope.

prompt_login would be an alternative name, if we need it.

[faho]

Alright, renamed to prompt_login. It's not perfect, but at least it doesn't have the host/hostname confusion.