MFC r306617

shutdown appeared as a standalone utility in 4.1BSD.
http://mail-index.netbsd.org/source-changes-d/2016/09/13/msg008686.html
http://mail-index.netbsd.org/source-changes-d/2016/09/14/msg008691.html
PR: 212552
Approved by: bcr (mentor)
Obtained from: NetBSD
Differential Revision: https://reviews.freebsd.org/D8105