The NIS backend regressed a while back when we reorganized the modules.

The import nis statements ended up importing our backend, rather than

Python's nis module. We only recently started hearing about this,

indicating that NIS authentication isn't widely used, but is still

used by some.
This change fixes the core issue by enabling

__future__.absolute_import, and enhances the code to be more testable

and address some old code quirks. A new unit test suite was added to

ensure that all NIS behavior works as expected.

Unit tests pass.