Make rootless privileged containers share the same tty devices as rootfull ones

[mupuf]

Until Podman v4.3, privileged rootfull containers would expose all the
host devices to the container while rootless ones would exclude
`/dev/ptmx` and `/dev/tty*`.

When 5a2405ae1b3a ("Don't mount /dev/tty* inside privileged containers
running systemd") landed, rootfull containers started excluding all the
`/dev/tty*` devices when the container would be running in systemd
mode, reducing the disparity between rootless and rootfull containers
when running in this mode.

However, this commit regressed some legitimate use cases: exposing
non-virtual-terminal tty devices (modems, arduinos, serial
consoles, ...) to the container, and the regression was addressed in
f4c81b0aa5fd ("Only prevent VTs to be mounted inside privileged
systemd containers").

This now calls into question why all tty devices were historically
prevented from being shared to the rootless non-privileged containers.
A look at the podman git history reveals that the code was introduced
as part of ba430bfe5ef6 ("podman v2 remove bloat v2"), and obviously
was copy-pasted from some other code I couldn't find.

In any case, we can easily guess that this check was put for the same
reason 5a2405ae1b3a was introduced: to prevent breaking the host
environment's consoles. This also means that excluding *all* tty
devices is overbearing, and should instead be limited to just virtual
terminals like we do on the rootfull path.

This is what this commit does, thus making the rootless codepath behave
like the rootfull one when in systemd mode.

This leaves `/dev/ptmx` as the main difference between the two
codepath. Based on the blog post from the then-runC maintainer[1] and
this Red Hat bug[2], I believe that this is intentional and a needed
difference for the rootless path.

Closes: #16925
Suggested-by: Fabian Holler <mail@fholler.de>
Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>

[1]: https://www.cyphar.com/blog/post/20160627-rootless-containers-with-runc
[2]: https://bugzilla.redhat.com/show_bug.cgi?id=501718

Does this PR introduce a user-facing change?

Rootless privileged containers will now mount all tty devices, except for the virtual-console ones (/dev/tty[0-9]+)

[mupuf]

@fho: I hope this wall of text makes sense!

[mupuf]

2 test failures seem to be unrelated to my changes (502 from docker.io in the int * tests), and no idea why the APIv2 tests failed :s

[fho]

@mupuf sounds good to me, great explanation and references!

[mheon]

LGTM

[edsantiago]

Submitted containers/buildah#4521 to try to fix the flake, but that's not going to help this PR. ITM all we can do is play the rerun game.

[vrothberg]

/lgtm

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[giuseppe]

this caused a regression with runc:

$ bin/podman --runtime /usr/bin/runc run --rm  --privileged alpine true
Error: OCI runtime error: /usr/bin/runc: runc create failed: unable to start container process: error during container init: error creating device nodes: open ~/.local/share/containers/storage/overlay/9c71e9c49cc103f6d02c7a2a2cbb81394f7fb109fcfd5034320c235825dd1c61/merged/dev/tty: no such device or address