* dll_init.cc (reserve_upto): removed (buggy, no longer needed)
        (release_upto): ditto.
        (dll_list::reserve_space): new function to reserve space needed by
        DLL_LOAD dlls early in the fork process. Exit cleanly and report
        to parent if fork failed due to clobbered address space.
        (dll_list::load_after_fork): rewritten. New version uses recursion
        to track reservations it makes while trying to make dlls land
        where they belong.
        (dll_list::load_after_fork_impl): see above.
        (dll_list::alloc): initialize image base field.
        * dll_init.h (struct dll_list): declare new functions.
        (struct dll): add member to track image base.
        * fork.cc (frok::child): call dll_list::reserve_space early, so we
        can retry if it fails.