Net::DNS remap compile-time Encode::XS ptr #305

Emit dlopen/dlfcn specific remap code in init2
  (most unix,mingw, not MSVC,solaris,...)
Currently this dynamic patch is specific to Net-DNS-0.67-74
but needs eventually be extended to other misbehaving modules.

compile-time XS symbols should not be stored as IV as the ptr will
be different when being executed.
initialize those pointers at run-time instead in
INIT blocks, and not with use constant.
See https://rt.cpan.org/Public/Bug/Display.html?id=94069