gcoap: accept resources in any order

[benpicco]

Contribution description

coap_match_path() returns the result of strcmp, so 0 is a match, non-zero give the order of the two strings.

For some reason GCoAP things the order is important - which leads to surprising results.

E.g. if you have

static const coap_resource_t _resources[] = {
    { "/fw", COAP_GET | COAP_MATCH_SUBTREE, gcoap_fileserver_handler, SUIT_FW_DIR },
    { "/cmd", COAP_PUT, _coap_cmd_handler, NULL },
    { "/log", COAP_PUT, _coap_log_handler, NULL },
};

The /cmd endpoint will always return 404.

Testing procedure

Issues/PRs references

[maribu]

Does the API doc need an update?

[kaspar030]

we could also check the order at the time the listener is registered, warn if it is not ordered, remember that and conditionally break. but hey, it won't matter until the number of endpoints gets larger than what we're doing now.

[benpicco]

Also changed it for coap_tree_handler()

[bergzand]

Is there a test where we can shuffle the resources around a bit?