warn about insufficient permissions

With this patch, if an unprivileged user has $HOME 700 or
750 and does

lxc-start -n c1

he'll see an error like:

lxc_container: Permission denied - could not access /home/serge.  Please grant it 'x' access, or add an ACL for t he container root.

(This addresses bug pad.lv/1277466)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
lxcapi-snapshot: don't snapshot directory-backed containers

Instead force a copy clone.  Else if the user makes a change
to the original container, the snapshot will be affected.
The user should first create a snapshot clone, then use
and snapshot that clone while leaving the original container
untouched.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>