Allow user to set custom DHCP name

src/net_builtin.h

@@ -47,6 +47,7 @@ struct mg_tcpip_if {
   mg_tcpip_event_handler_t fn;     // User-specified event handler function
   struct mg_mgr *mgr;              // Mongoose event manager
   struct mg_queue recv_queue;      // Receive queue
+  char dhcp_name[12];              // Name reported to DHCP, "mip" if unset
   uint16_t mtu;                    // Interface MTU
 #define MG_TCPIP_MTU_DEFAULT 1500