options.py: better support for explicit no-* options.

options.py: better support for explicit no-* options.

If a declared option name starts with no-xxx, then the 'xxx' option starts
off set to True by default, so that no-xxx is False by default, so that
passing --no-xxx as an option will have the desired effect of setting
--no-xxx=True (and thus --xxx=False).

Previously, trying to list a --no-xxx option in the argument list would
trigger an assertion failure.

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