dlopen: switch to new pathfinder class

dlopen: switch to new pathfinder class

Instead of find_exec, without changing behaviour use new pathfinder
class with new allocator_interface around tmp_pathbuf and new vstrlist
class.
* pathfinder.h (pathfinder): New file.
* vstrlist.h (allocator_interface, allocated_type, vstrlist): New file.
* dlfcn.cc (dlopen): Avoid redundant GetModuleHandleExW with RTLD_NOLOAD
and RTLD_NODELETE.  Switch to new pathfinder class, using
(tmp_pathbuf_allocator): New class.
(get_full_path_of_dll): Drop.