Bluetooth: Add mgmt_pair_device command

This patch adds a new mgmt_pair_device which can be used to initiate a
dedicated bonding procedure. Some extra callbacks are added to the
hci_conn struct so that the pairing code can get notified of the
completion of the procedure.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>