mDNS : implement service subtype to the LwIP mDNS application

[Abdel-dev-tech]

There are a lot of service types that are not in the standard (RFC 2782) as "_printer" for example. And in some circumstances in order to advertise a particular service or to be detected by other mdns applications that filter only some particular types, we need to use subtypes for that purpose.
The following pull request proposes an implementation of a service subtype in the LwIP mDNS application.
The solution is based on writing an answer of a subtype service PTR RR into the outpacket. In order to do that I made the following changes :

1. Update the service structure to contain a table of subtypes ( because a service can have many subtypes)
2. Update the mdns_resp_add_service() function to initialize the subtypes of a service
3. Add mdns_add_service_subtype_ptr_answer() function that writes answer of the subtype service PTR RR to outpacket

Useful :
To initialize a service with two subtypes we need to

• update the maximum subtype services number to 2 or more

#define MDNS_MAX_SERVICES_SUBTYPES 2

• Call the function mdns_resp_add_service as follows

char *subtype[]={"_iot_node","_printer"};

mdns_resp_add_service(gnetif, "ServiceTest1", "_rtsp", DNSSD_PROTO_TCP, 9090, srv_txt, NULL,subtype,2);

For testing purpose, for example to browse only "_printer" subtypes we can use the following command line on Linux (not that the subtypes are not displayed in the avahi graphical interface):

avahi-browse _printer._sub._rtsp._tcp