Bluetooth: Add management command for enabling Secure Connections

The support for Secure Connections need to be explicitly enabled by
userspace. This is required since only userspace that can handle the
new link key types should enable support for Secure Connections.

This command handling is similar to how Secure Simple Pairing enabling
is done. It also tracks the case when Secure Connections support is
enabled via raw HCI commands. This makes sure that the host features
page is updated as well.

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