issue #305: defer stash init of AMT magic

Encode::XS is dynaloaded and has AMT magic: TIEHASH
WIP
t/issue305.t: for the Encodings special-case #305

add testcase and document special-solution via dlsym
(not coded in the compiler yet)
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
fix for <5.14, push arrayref

also fix MANIFEST
More XS remapping: iso8859_1, null, ascii_ctrl

and Net::LibIDN not yet verified.
Warn on possibly more needed XPVMG remapped ptrs
comments and Changelog
seperate optional from mandatory init2 remaps

die with mandatory, only warn with optional.