Start perl5200delta

just the template for now
List broken modules from #119125 in perl5200delta

so that we can track these all in one spot and close the various RT
tickets as we go.

I am only listing ‘high-profile’ modules, meaning those that have
at least 5 dependents (an arbitrary number picked by Jesse Vincent
some time ago).
Add perl5200delta to MANIFEST
podcheck.t: Skip perl5200delta

I would have to change the header to something other than
‘perldelta’, even though that is what the final document
will have.

I would also have to start adding modules to the known modules
list which will not have to be there in the final release.

That is more work than necessary, so just skip this file for now.
Don’t give unavailability warnings about our vars

Commit ce0d59fdd1c started using NULL to indicate nonexistent array
elements.  Since pads are AVs, they are filled with NULLs initially,
rather than &PL_sv_undef.

For ‘our’ vars, the pad entry is never actually touched.  Only one
piece of code was inspecting it, namely S_cv_clone_pad.  &PL_sv_undef
just happens to pass the checks that make sure the var is not stale.
However, we really should not be checking that at all if this is an
‘our’ var.

Even if we change ‘our’ vars back to having a &PL_sv_undef pad
entry, this fix should stay, as it makes the code clearer and makes
S_cv_clone_pad more robust.