Split out UserAuthenticate

[jessepeterson]

Also add tests and a CLI option to turn on "zero-length" DigestChallenge mode (-ua-zl-dc).

Per the updated Operations Guide docs:

By default NanoMDM will response to the UserAuthenticate message with an HTTP 410 response effectively declining management of the MDM user. Enabling this option turns on the "zero-length" Digest Challenge mode where reply with an empty Digest Challenge to enable management each time a client enrolls. See also Apple's discussion of UserAthenticate for more information.