ssdp: send a full byebye + alive set when a new interface comes online

If an interface comes online after startup, we really need to send a
complete byebye + alive set on that interface, but we don't want to do
it for other interfaces.  So do it as the interfaces come up, and
skip it for interfaces that existed before the reload.

upnpglobalvars.h

@@ -215,10 +215,9 @@ extern char pnpx_hwid[];
 #endif
 
 /* lan addresses */
-/* MAX_LAN_ADDR : maximum number of interfaces
- * to listen to SSDP traffic */
 extern int n_lan_addr;
 extern struct lan_addr_s lan_addr[];
+extern int sssdp;
 
 extern const char *minissdpdsocketpath;