Bluetooth: Create a unified authentication request function

This patch adds a single function that's responsible for requesting
authentication for outgoing connections. This is preparation for the
next patch which will add automated name requests and thereby move the
authentication requests to a different location.

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