Add explicit initialization for all PlannerGlobal fields

While adding a new field to PlannerGlobal in another patch, I noticed
that although most fields are explicitly initialized, a few are not.
This doesn't cause any functional issues, since makeNode() zeroes all
fields by default.  However, the inconsistency stood out to me, so I
wrote the attached patch to explicitly initialize the remaining fields
for clarity and consistency.

Does this seem worthwhile?  Or should we simply rely on makeNode() for
zero-initialization and consider this unnecessary?

Thanks
Richard