To not be confused with arg_nogroups, as in the vast majority of cases
drop_privs is called with either 0 or 1 rather than arg_nogroups.  The
rename makes it clearer that what the parameter does is to drop all
groups without exception, unlike arg_nogroups, which may have certain
groups be kept.

This amends commit 11418a4 ("dns fixes", 2019-10-31).

fwarning already prints "Warning: " at the beginning.

Kind of relates to commit 6ddedeb ("Make nogroups work on nvidia
again", 2021-11-29) / PR netblue30#4725, which removed code affected by this.

Command used to find the duplicates:

    git grep -i -F 'fwarning("Warning:' -- src

This amends commit b828a90 ("Keep audio and video groups regardless of
nogroups", 2021-11-28) from PR netblue30#4725.

The commit above did not change the behavior (the groups are still not
kept).  With this commit, it appears to work properly:

    $ groups | grep audio >/dev/null && echo kept
    kept
    # with check_can_drop_all_groups == 0
    $ firejail --quiet --noprofile --nogroups groups |
      grep audio >/dev/null && echo kept
    kept
    # with check_can_drop_all_groups == 1
    $ firejail --quiet --noprofile --nogroups groups |
      grep audio >/dev/null && echo kept
    $

Add a new check_can_drop_all_groups function to check whether the
supplementary groups can be safely dropped without potentially causing
issues with audio, 3D hardware acceleration or input (and maybe more).
It returns false if nvidia (and no `no3d`) is used or if (e)logind is
not running, as in either case the supplementary groups might be needed.

Note: With this, the behavior from before netblue30#4725 is restored on (e)logind
systems (when not using nvidia), as it makes the supplementary groups
always be dropped on such systems.

Note2: Even with the static variable, these checks still happen at least
twice.  It seems that it happens once per translation unit (and I think
that it may happen more times if there are multiple processes involved).

This also amends (/kind of reverts) commit 6ddedeb ("Make nogroups
work on nvidia again", 2021-11-29) from PR netblue30#4725, as it restores the
nvidia check from it into the new check_can_drop_all_groups function.