Re: Add pg_get_injection_points() for information of injection points

> Would it be useful to put the logic of the above function under #define
> USE_INJECTION_POINT.  This approach would make it possible to
> distinguish between cases where no injection points are attached and
> instances where the build does not support injection points.

For this one, I've found that InjectionPointList() was the incorrect
bit: we can make it issue an elog(ERROR) if !USE_INJECTION_POINTS.
This way, none of its callers will be confused between the case of a
NIL List meaning either !USE_INJECTION_POINTS or that there are no
points attached if the build uses USE_INJECTION_POINTS.