os-posix: set groups properly for -runas

os-posix: set groups properly for -runas

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: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>