Rework quiet, debug and normal console output

[rusty-snake]

We have multiple issues about how we print something to the console, which all go to the same code. Therefore I make this meta-issue.

• Error: invalid --debug command line option if quiet-by-default is set in firejail.config #4168
  Fixed in 65b7cd7

• no way to selectively disable quiet-by-default in firejail.config #3125
  Fixed in 65b7cd7

• "ignore quiet" does not work in override (.local) files #3503

• Option --quiet suppresses important warnings #3354

• Where to print debug messages (stdout or stderr)? #3749

• Seccomp list output goes to stdout instead of stderr #4328

• Add visual cue for firejail outputs #2322

There are some more which aren't handled here like:

• Print better error in the case of ownership issues #2743
  Fixed in 9fd3560
  and 9a84e4d

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Good to read:

• Error: invalid --debug command line option if quiet-by-default is set in firejail.config #4168 (comment)

IMHO we should only print warnings and errors to stderr by default and be quiet for everything else. The current quiet would be then a hard-queit if a program really need full ownership of stderr. In addition a --info/--verbose/--debug=1/--debug=little/whatever will show the current default output.

All the warnings/errors should make clear that they are printed by firejail (e.g. firejail:Error:).

For the debug output it would be enough to print something like

--- end of firejail output ---

[glitsj16]

All the warnings/errors should make clear that they are printed by firejail (e.g. firejail:Error:).

This reminds me of #3325, which never really got of the ground. @topimiettinen mentioned a possibility to improve errExit() and use it everywhere with a prefix. IMO that would solve this part of the issue. Just adding it here for future reference.

[slowpeek]

I'm not sure is there a single reason to use stdout in firejail. It is a wrapper, hence it should not touch neither stdin nor stdout.