- `hashmap_insert()` no longer takes key as argument, and
takes the pointer to be associated with the entry as argument.
- Rename `hashmap_entry_delete` -> `hashmap_delete`.
- Make `user_data` the first argument for `hashmap_walk_func`.
- Other misc renames.
That part of code in configure.ac look stupid, remove it.
If we _do_ try to run BookmarkFS on such an exotic platform with
non-zero null pointers, the breakage should be detected by the tests
(if it can run or even build at all).
