Fix failure due to bind-mount through /proc
Fix failure due to bind-mount through /proc

The kernel won't allow us to bind-mount stuff through /proc.
Since the main reason for my previous change was to fix /dev handling,
lets just whitelist /dev as safe to read from within the snap, which
should be fine so long as distros use devtmpfs.

Closes #3968

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

lxc/lxd-pkg-ubuntu,lxc/lxd-pkg-ubuntu,lxc/lxd-pkg-ubuntu