lxc.slackare.in is a modified version of the file shipped in
Slackware64-current, archived as lxc.slackware.in.orig

Rationale: see this thread:
http://www.linuxquestions.org/questions/slackware-14/%5Brequest-for-comments%5D-do-we-still-need-cgmanager-4175599676/

To use the Slackware template to build an unprivileged container,
read the instructions in /usr/share/lxc/templates/lxc-slackware

Didier Spaier <didier~at~slint~dot~fr> 10 August 2017