Andrew Griffiths reports that -runas does not set supplementary group
IDs.  This means that gid 0 (root) is not dropped when switching to an
unprivileged user.

Add an initgroups(3) call to use the -runas user's /etc/groups
membership to update the supplementary group IDs.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: NChris Wright <chrisw@sous-sol.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>