Bluetooth: Add support for building pairing commands

Before we are able to do a proper exchange of pairing parameters,
we need a unified way of building pairing requests and responses.

For IO Capability we use the value that was set by userspace,
using the management interface.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>