This is a follow up for 3f956201.
Before this commit, it was possible to create a shared string which
shares with another shared string by passing a frozen shared string
to `str_duplicate`.

Such string looks like:

```
 --------                    -----------------
 | root | ------ owns -----> | root's buffer |
 --------                    -----------------
     ^                             ^   ^
 -----------                       |   |
 | shared1 | ------ references -----   |
 -----------                           |
     ^                                 |
 -----------                           |
 | shared2 | ------ references ---------
 -----------
```

This is bad news because `rb_fstring(shared2)` can make `shared1`
independent, which severs the reference from `shared1` to `root`:

```c
/* from fstr_update_callback() */
str = str_new_frozen(rb_cString, shared2);  /* can return shared1 */
if (STR_SHARED_P(str)) { /* shared1 is also a shared string */
    str_make_independent(str);  /* no frozen check */
}
```

If `shared1` was the only reference to `root`, then `root` can be
reclaimed by the GC, leaving `shared2` in a corrupted state:

```
 -----------                         --------------------
 | shared1 | -------- owns --------> | shared1's buffer |
 -----------                         --------------------
      ^
      |
 -----------                         -------------------------
 | shared2 | ------ references ----> | root's buffer (freed) |
 -----------                         -------------------------
```

Here is a reproduction script for the situation this commit fixes.

```ruby
a = ('a' * 24).strip.freeze.strip
-a
p a
4.times { GC.start }
p a
```

 - string.c (str_duplicate): always share with the root string when
   the original is a shared string.
 - test_rb_str_dup.rb: specifically test `rb_str_dup` to make
   sure it does not try to share with a shared string.

[Bug #15792]

Closes: https://github.com/ruby/ruby/pull/2159