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.
generalize dlsym to XS_DynaLoader_dl_find_symbol

should now work on other non-d_dlopen platforms also.
relax ptr detection: U?L+$, fixes solaris (ULL)
replace $can_native with $HAVE_DLFCN_DLOPEN
remap improvements via SVp_POK of symbol-name

I used a private patch for Encode 2.48 to store the symbol name in the PVX
slot of the enc IV with SVp_POK to be able to thaw that symbol via dlsym ar run-time
from a frozen image.
This can be uses as perl5 API to store all external XS symbols when they are stored at
compile-time and not properly restored at run-time. I.e. when being initialized compile-time
and used in a lexical or constant.
mark package with use constant

svop constant are generated by use constant symbols being used.
i.e. ASCII->method. Fixes even the generic case t/testc.sh 305