options.py: clean up handling of --no-* options.

options.py: clean up handling of --no-* options.

The particular bug that triggered this (in a project other than bup) was of
the form:

	n,no-stupid       don't be stupid

Where it would actually end up setting stupid=1 by accident, and -n would
mean --stupid, not --no-stupid.  As part of fixing it, you can now also do
this:

	n,no-stupid,smart   don't be stupid (ie. be smart)

and it'll work as it should: n == smart == no-stupid == not stupid.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>