Stop warning on safe supplementary group clean

[kmk3]

When nogroups is used, the following warning may be issued (potentially
multiple times, as drop_privs may be called more than once):

Warning: cleaning all supplementary groups

But the warning is being shown even when it seems that all supplementary
groups can be safely dropped (and are thus dropped), which is likely a
common scenario. This commit prevents the warning from being printed in
that case, making it so that it is only shown in the non-happy paths (as
was the case on firejail 0.9.66).

Misc: The added code was copied from drop_privs.

This amends commit 7abce0b ("Fix keeping certain groups with
nogroups", 2021-11-30) / PR #4732.

Kind of relates to #4930.

[glitsj16]

Can't really be sure on the coding side of this PR, but it definately deserves full credit for the effort to do something about these superfluous (hence potentially confusing) warnings: +1!

[kmk3]

@glitsj16 left a comment:

Can't really be sure on the coding side of this PR, but it definately
deserves full credit for the effort to do something about these superfluous
(hence potentially confusing) warnings: +1!

Thanks; sorry for taking so long to fix this.

This is the one part of the code that I changed without having a good enough
understanding of it and now it's rather messy, but I'm working on fixing that.

There's one more notable bugfix to post and a WIP refactor/unification of group
handling, which should simplify some things a lot.

Though I can't see an easy fix for one key part, which is the fact that
clean_supplementary_groups may be called during argument parsing but
(since #4725/#4732) it depends on arg_ variables.

If all arguments/profile entries were parsed before doing anything "important",
I think it would make using arg_ variables a lot safer/more robust. I
remember someone suggesting something like this in an issue (or maybe in a
comment), but I can't find it at the moment.

[netblue30]

all in, thanks!