19272: gcoap: Do not send responses from multicast addresses r=benpicco a=chrysn
### Contribution description
Since https://github.com/RIOT-OS/RIOT/pull/18026, CoAP requests to multicast addresses (eg. `ff02::1`) came back from that exact address, which Linux rightfully just drops.
The fix uses the existing multicast check from https://github.com/RIOT-OS/RIOT/pull/17978 (thanks `@benpicco` for making me write this as dedicated function, I just had to generalize it removing one struct layer), and foregoes setting the source address when responding to multicasts.
### Testing procedure
* Run the gcoap example
* Send a CoAP request to a multicast address RIOT listens to, eg. `./aiocoap-client coap://'[ff02::1%tapbr0]'/.well-known/core --non`
Before, this got no response (while you see it arrive on wireshark). After, you get a correct response with two lines of note:
```
WARNING:coap:Sending request to multicast via unicast request method
Response arrived from different address; base URI is coap://[fe80::3c63:beff:fe85:ca96%tapbr0]/.well-known/core
```
(The former is aiocoap telling us that we're not using the nonexistent multicast API so it's really more of an anycast, the latter is useful factual information).
Co-authored-by: chrysn <chrysn@fsfe.org>
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