Change LOADER_PATH, support native load extensions, fix load readline

. at the begin of the LOADER_PATH is a security risc, put it at the end.
Search in lib and PREFIX/lib/potion before.
Search only for native load extensions: .so/.dylib/.dll
Store extensions for exe, load (.dylib), dll (.bundle) and lib in config.h, also provide them
in config.sh.
Move lib/readline/readline.so to the searchable lib/readline.so, fix uninstalled interactive mode.
load .pnb before .pn without timestamp check (yet).
Add .i target for CC -E -c preprocessed files.