Change su behavior to prompt target user unless the user is root

[mbish]

There have been a number of requests to alter the su behavior for Duo Unix to prompt the target user rather than the calling user.

[dnorthup-ums]

Is there any intent to make this configurable? We'd like to allow developers to have Duo-protected su / sudo access to the user that many of our HCM & Financials integrations run as, and the way this is currently implemented we'd be prevented from doing so in any reasonable manner. (I agree that protecting individual accounts, as this change does, should be required, so I don't want to see it reverted.)