quoted

We sort of have one. It's called capable(). Just needs extending to cover
anything else you care about, and probably all the numeric constants
replacing with textual names.

Except that it's all backwards: these are things that default to *on*,
and people might want them to turn off.  capable() is totally fscked
if you want otherwise unprivileged users to carry capabilities around

The userspace API is, but capable() as a userspace API and capable() as
an in kernel check are only connected by history.

For the in kernel part you can either teach everyone another disjoint API
or we can have a single API in kernel for saying "is XYZ allowed".