Rename all .h files to .hpp.

.h is for C headers, .hpp for C++ ones.
Correct all the includes for the new .hpp files
Rename config.h to config.hpp
Remove ENABLE_NEW_LOADER macro from config.hpp, it’s useless now
Remove now useless modelines
Replace _H guard macros with _HPP ones
Reset mruby-pcre-regexp before patching it.

This allows multiple building without having to revert the patch
manually first.