Fix for chroot not getting supplemental groups

linux chroot doesn't get the supplemental groups before dropping privileges. As such, we need to pull it from /etc/group and tweak it so that we can send them to chroot. 

Original report came from @spuder on irc -- here is his GIST report: 

https://gist.github.com/spuder/a1c3c7d10ce129507858

Spuder tested this and reported it working. Should work on other systems that use sysv init.