Pass in the full arguments to ExtUtils::Constant::ProxySubs

Pass in the full arguments to ExtUtils::Constant::ProxySubs
Prototype all the missing constants, so that parsing doesn't depend on
the phase of the moon. (Well, the system headers)
Add an option for making missing constants generate errors at read time
(which can be during constant folding). This isn't the default.

p4raw-id: //depot/perl@26506