This change renames `coap_join_mcast_group` to `coap_join_mcast_group_intf`
and adds an extra `const char *ifname` parameter to specify the interface
name.
A `coap_join_mcast_group` macro is defined to maintain backwards API
compatibility.
`coap_is_mcast` has been modified to recognize IPv6-mapped IPv4 multicast
addresses. Without this `coap_network_send` treats these addresses as
unicast which makes `semdmsg` fail with EINVAL.
Examples have been updated to use the new call.
Szabolcs Szekelyi
42b2f03470