Fix external entity loader

[robb2040]

I found the following issues with externalEntityLoader(...) when trying to use it for a single XML Schema parse without affecting later other parses:

• externalEntityLoader(...) does not take effect when
  • ext_ent_handler has been passed in a previous parser instantiation - t/49global_private_extent.t
  • no_network has been passed in a previous parser instantiation - t/49global_extend_after_no_network.t
• after calling externEntityLoader it is not possible to clear it: setting it to undef
  • still keeps LibXML_load_external_entity installed, which e.g. does not check no_network - t49/global_extent_reset.t
  • still hides an ext_ent_handler passed to a later parser instantiation - t49/global_private_extent.t test 4
• the first call to externalEntityLoader sometimes causes a segfault (at least in some cases when the return value is used)

and would suggest this PR as a solution:

• Make externalEntityLoader and ext_ent_handler independent, including their storage of 'old handler', with externalEntityLoader overriding ext_ent_handler like now
• Interpret externalEntityLoader(undef) as 'fully undo setting externalEntityLoader'
• Return &PL_sv_undef instead of NULL when no handler was previously set

Thanks for XML::LibXML!