enum: Add compile-time check for missing strings

enum: Add compile-time check for missing strings

If strings are missing (e.g. because the last value of a range changed
unknowingly or adding a string was simply forgotten) compilation will
now fail.

This could be problematic if the upper limit is out of our control (e.g.
from a system header like pfkeyv2.h), in which case patches might be
required on certain platforms (enforcing at least, and not exactly, the
required number of strings might also be an option to compile against
older versions of such a header - for internal enums it's obviously
better to enforce an exact match, though).