MFC r322677: pw usermod: handle empty secondary group lists (-G '')
"pw usermod someuser -G ''" is supposed make sure that someuser
doesn't have any secondary group memberships.
Previouly it was a nop because split_groups() only intitialised
"groups" if at least one group was specified. As a result the
existing secondary group memberships were kept.
PR: 221417
Submitted by: Fabian Keil
Approved by: re (kib)
Obtained from: ElectroBSD
Relnotes: yes
git-svn-id: svn://svn.freebsd.org/base/stable/10@323333
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