Move pp_-specific code out of core_prototype

Move pp_-specific code out of core_prototype

Commit b8c38f0a2a65 refactored pp_prototype by moving much of its
code to a new function in op.c, called core_prototype.  This served
two purposes: (1) to allow the code to be simplified, which required
the use of static functions in op.c, and (2) to allow the &CORE::subs
feature to share the same code.

But some code was moved to core_prototype which, in hindsight, did not
need to be moved, such as the ‘Can’t find an opnumber’ message.

This commit moves that code back to pp_prototype, resulting in a sim-
pler (and possibly faster, at least for &CORE::subs) core_prototype.