Allow users to disable custom console background color

[qdeslandes]

When mkosi creates a new console with systemd-pty-forward, it sets a custom (dark) color, which might not play nice depending on your terminal theme.

Introduce the WithConsoleBackground= [Runtime] option to disable with behaviour. Custom console background is enabled by default, so the current behaviour is not modified.

[behrmann]

I agree this would be useful, but mkosi is not the right place to fix this, but this should preferably go into systemd-pty-forward. systemd-run and friends already take an environment variable SYSTEMD_RUN_SHELL_PROMPT_PREFIX, so something like SYSTEMD_PTY_FORWARD_BGCOLOR would make sense, which we could then check for and not set our own colours.

[DaanDeMeyer]

systemd/systemd#39382 will do the trick by allowing you to set the SYSTEMD_TINT_BACKGROUND=0 environment variable globally to disable color tinting.

[DaanDeMeyer]

We can just check the same environment variables in mkosi, so check for $SYSTEMD_TINT_BACKGROUND and $SYSTEMD_ADJUST_TERMINAL_TITLE in config.environment and os.environ. If both are not set we can skip using systemd-pty-forward completely.

[qdeslandes]

I've modified the logic to check apply --background if SYSTEMD_TINT_BACKGROUND is not 0 in mkosi's environment variable or the global environment variables. Same for --title.

systemd-pty-forward is still used if both the custom title and custom background are disabled as the --read-only flag is passed in some cases, it makes the logic simpler to use it by default.