With this option enabled (new), the multiresults returned by a stored C
function via iproto aren't wrapped in the additional msgpack array (old).

Due to new behaviour some renames are performed:
* `port_c_dump_msgpack()` -> `port_c_dump_msgpack_wrapped()`, since this
  is dump format with additional msgpack array encoded.
* `port_c_dump_msgpack16()` -> `port_c_dump_msgpack()`, since this
  format is now the default new format of a msgpack dump.

The behaviour of the C port msgpack dumping depends on the
`c_func_iproto_multireturn` option:
* uses `port_c_dump_msgpack()` if set to true (new),
* uses `port_c_dump_msgpack_wrapped()` otherwise (old).

Needed for #4799

@TarantoolBot document
Title: Document `c_func_iproto_multireturn` compat option

Please create a documentation page for the new compat option:
https://tarantool.io/compat/c_func_iproto_multireturn

In the new behaviour, the multiresults returned by a stored C function
via iproto aren't wrapped in the additional msgpack array (old).

```
tarantool> compat.c_func_iproto_multireturn = 'old'
---
...

tarantool> net_box.connect(box.cfg.listen):call('myclib.cfunc')
---
- [true, -1]
...

tarantool> compat.c_func_iproto_multireturn = 'new'
---
...

tarantool> net_box.connect(box.cfg.listen):call('myclib.cfunc')
---
- true
- -1
...

```

The new behaviour is consistent with the local call of the function
via `box.func`:

```
tarantool> box.func['myclib.cfunc']:call()
---
- true
- -1
...

```

Assume you have a stored C function that returns values like the
following:

```c
char *position = mp_encode_bool(buffer, true);
box_return_mp(ctx, buffer, position);
/* ... */
position = mp_encode_int(buffer, -1);
box_return_mp(ctx, buffer, position);
```

If you want to preserve the format of the returned array for your C
functions, when the `c_func_iproto_multireturn` option is set to "new",
you should add the additional wrapping, like the following:

```c
char *position = mp_encode_array(buffer_with_results, n_results);
position = mp_encode_bool(position, true);
/* ... */
position = mp_encode_int(position, -1);
box_return_mp(ctx, buffer_with_results, position);
```

The amount of `box_return_mp()` calls indicates the number of values to
be returned.

Also, you should update its usage via `box.func` if there is any.