Bluetooth: Open management interface for untrusted users

Until now the management interface was restricted to CAP_NET_ADMIN. With
this change every user can open the management socket. However the list
of commands is heavily restricted to getting basic information about the
attached controllers. No access for configuration or other operation is
provided. The events are also limited. This is done so that no keys can
leak or untrusted users can mess with the Bluetooth configuration.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>