Bluetooth: SMP: Add option to disallow unauthenticated re-pairing

Disallows all unauthenticated pairing attempts made by the
peer where an unauthenticated bond already exists.
This would enable cases where an attacker could copy the peer device
address to connect and start an unauthenticated pairing procedure
to replace the existing bond. Now in order to create a new bond the old
bond has to be explicitly deleted with bt_unpair.

Added option to disable this rule in order to maintain backwards
compatibility in case this behavior is accepted.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>