useraddcern: default to local homedir

Jan Ivenrequested to merge
iven/useraddcern:noafsbydefault into master

useraddcern still defaulted to creating AFS homedirectories on desktops, this is deprecated since ca 2017. This patch makes local homedirectories the default, with a new flag --directory to get back the old behaviour (e.g for small-scale desktop clusters).

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