As we add more features, the current code is insufficient.
1. Keep an array of single feature bits, for easy switching on and off.
2. Create feature_offered() which checks for both compulsory and optional
variants.
3. Invert requires_unsupported_features() and unsupported_features()
which tend to be double-negative, all_supported_features() and
features_supported().
4. Move single feature definition from wire/peer_wire.h to common/features.h.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
46cc7c281e