This introduces a new type features_t that exposes feature flags. The intent
is to allow a deprecation/incremental adoption path. This is not a general
purpose configuration mechanism, but instead allows for compatibility during
the transition as features are added/removed.
Each feature has a user-presentable short name and a short description. Their
values are tracked in a struct features_t.
We start with one feature stderr_nocaret, but it's not hooked up yet.
