better support for $COLORTERM (#36770)

I recently noticed that our serial/VM terminals did not get fedora's
color shell prompt, nor got color support in "ls".

I spend a bit of time investigating and it's all a bit of a mess. If we
don't have any idea what kind of terminal we are talking to via serial
or hypervisor console then we so far just set TERM=vt220 as a reasonable
fallback: vt220 is quite universally defined in terminfo/termcap, and it
supports pageup/pagedown (unlike vt100).

However, real vt220 DEC terminals did not support color, and hence
termcap/terminfo says "no color, sorry". Which sucks, but actually
neither coreutils' "ls" (via `dircolors`) nor fedora's color shell
prompt actually care for termcap/terminfo. So why don't we get color?

In the coreutils case: it has it's own mini-database of terminals. A
very skewed one, where TERM=vt100 enables colors (and DEC vt100
definitely never ever had color support!), but vt220 does not. However,
what it actually does is check $COLORTERM. If that's set then it would
enable color.

In the fedora color prmpt case: it tries to derive color support by
looking for the word "color" in $TERM. Horrible hack if you ask me...

In order to make things better I did a bunch of things:

1. I think the idea of actually having a fully correct and up-to-date
termcap/terminfo database is kinda illusionary these days. But
apparently regarding color support $COLORTERM kinda took it place.
coreutils cares, and systemd itself cares too. To some point at least:
we consume it to determine color support, but we never propagate it in
nspawn, run0 and so on. So this PR fixes that.

2. Also, we are kinda stuck with vt220 I guess as default fallback for
serial terminals. But let's tweak it, and set $COLORTERM=truecolor as
default too. this means we default to a vt220 terminal, but with color.
Which is an ahistorical thing to do, but I think it's the best way out.

3. I also filed a bug against util-linux asking them to treat $COLORTERM
like $TERM, and let it propagate from getty into login shell:
https://github.com/util-linux/util-linux/issues/3463 – With that we
should get color support in ls by default now.

4. I also asked coreutils to treat vt220 the same as they already treat
vt100 and simply do color, even if though that's ahistorical:
https://github.com/coreutils/coreutils/issues/96

5. I then asked the fedora color prompt package to check $COLORTERM:
https://bugzilla.redhat.com/show_bug.cgi?id=2352650

6. I also asked the fedora ssh package to propagate $COLORTERM to remote
hosts by default, like they already cover $TERM. terminal emulators set
both these days generally, hence this would make sense.
https://bugzilla.redhat.com/show_bug.cgi?id=2352653

7. while at it, I figured it makes sense to not only propagate/consume
$COLORTERM at the same time as $TERM, but also consider $NO_COLOR. In
contrast to $COLORTERM for which no spec seems to exist, that one
actually does have a spec: https://no-color.org/

It might make sense for those interested in other distros than Fedora to
maybe ask for similar changes for their ssh and color shell prompt
packages (if they have something coresponding).