Do not instantiate template ctors to make it take an ioctortype arg.

Fixes ROOT-5547.
Revert "Do not instantiate template ctors to make it take an ioctortype arg."

This reverts commit 223f631d327cd08540263c985d127cebc3255f9f.
Philippe and I violently disagree on whether template <typename T> Klass(T) should be taken into account when matching the RootIOCtor type.
I find this intricate, close to random (because it depends on what rootcling sees or whether the instantiation has been moved to a source file), non-obvious for such an explicit request.
Philippe finds it handy to invade / kidnap "unintrusively" existing templated c'tor signatures.
I request an explicit c'tor overload.
Philippe thinks that's too much of a requirement; he could not find a use case for the templated c'tor, though.
increase default xml buffer size to avoid frequent problesm with that.. sorry, don't know yet how to make that easily more flexible
Add a way to directly detect Constructor and Destructor (and convertor)

Add TFunction::ExtraProperty (and TClingMethodInfo::ExtraProperty)
and the enum values:
   kIsConstructor
   kIsConversion
   kIsDestructor

[The set of values use for Property, see TDictionery EProperty
uses all 32 bits of the typical enum underlying storage]