add old config and unprivileged to check_ct_modify_config_perm

add old config and unprivileged to check_ct_modify_config_perm

we'll need that for checking the features more granularly
for it to work correctly, we have to move the permission checks
into the 'lock_config' sub, since we now also need to check the current
config and it could change between the permission check and the lock

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>