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