Return false (i.e. "did not match") but no error when evaluating a match
expression against a "flag" type feature (which don't have any
associated value, just the name) if a MatchOp that never matches is
used.
This is preparation for supporting multi-type features, i.e. one
feature, like "cpu.cpuid", having e.g. "flag" and "attribute" type
features.
