lxc-ls: Cache groups and show bygroup in autostart

This makes sure we only query lxc.group once and then reuse that list
for filtering, listing groups and autostart.

When a container is auto-started only as part of a group, autostart will
now show by-group instead of yes.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Revert "cgfs: don't mount /sys/fs/cgroup readonly"

This reverts commit 8d783edcae3723a0106d75e1ff31b016e8b1d02c.