* dll_init.cc (dll_list::find_by_modname): new function to search
        the dll list for a module name only (no path).
        (dll_list::alloc): Initialize newly-added members of struct dll.
        (dll_list::append): new function to factor out the append
        operation (used by dll_list::topsort).
        (dll_list::populate_deps): new function to identify dll dependencies.
        (dll_list::topsort): new function to sort the dll list
        topologically by dependencies.
        (dll_list::topsort_visit): new helper function for the above.
        * dll_init.h (struct dll): added new class members for topsort.
        (struct dll_list): declare topsort-related functions.
        (struct pefile): allows simple introspection of dll images.
        * fork.cc (fork): topsort the dll list before forking. When the
        child loads each dll, all dependencies are already loaded and will
        no longer risk being pulled in unexpectedly.