Signature of a function which acquires a credential object.

• cred: The newly created credential object.
• url: The resource for which we are demanding a credential.
• username_from_url: The username that was embedded in a "user@host" remote url, or NULL if not included.
• allowed_types: A bitmask stating which cred types are OK to return.
• payload: The payload provided when specifying this callback.
• returns 0 for success, < 0 to indicate an error, > 0 to indicate no credential was acquired