scripts: config-lib improve setup_sd_user

[bruno-at-bareos]

With the current code, usermod -G .... call is done quoted generating a warning on several distribution when installing bareos-storage package.

Running scriptlet: bareos-storage-22.0.4~pre51.ab585a79e-44.el8.x86_64                                                                                                                                                  19/39
/usr/lib/bareos/scripts/bareos-config-lib.sh: line 370: usermod -G bareos,tape,disk: command not found
Warning: failed: usermod -G bareos,tape,disk bareos

This PR propose to use simpler command construction by calling usermod -a -G group,group in all case.
usermod doesn't failed if the user already belong to the group, and with -a we preserve the already defined group.

• Use flag -a with -G on usermod command.
• Remove unneeded code for additional groups.

OP#5450

I propose to also create a backport to 22 for that one.

Result Before

id bareos
uid=988(bareos) gid=984(bareos) groups=984(bareos)

Result After

 id bareos
uid=988(bareos) gid=984(bareos) groups=984(bareos),6(disk),33(tape)

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[arogge]

I'm really sorry I'm bothering you every time. But you really have a talent for picking the worst shell script that has the most potential for improvement...

[arogge]

This looks great! I'll try it and discuss with @pstorz if we should delay 22.1.0 to include this.