Make the Distribution() constructor forgiving of unknown keyword

Make the Distribution() constructor forgiving of unknown keyword
arguments, triggering a warning instead of raising an exception.  (In
1.5.2/2.0, it will print to stderr.)

Bugfix candidate for all previous versions.  This changes behaviour,
but the old behaviour wasn't very useful.  If Distutils version X+1
adds a new keyword argument, using the new keyword means your setup.py
file won't work with Distutils version X any more.