Bluetooth: Map sec_level to link key requirements

Keep the link key type together with connection and use it to
map security level to link key requirements. Authenticate and/or
encrypt connection if the link is insufficiently secure.

Signed-off-by: Waldemar Rymarkiewicz <waldemar.rymarkiewicz@tieto.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>