Use DLT_RAW for DL_IPNET devices; that's their default behavior.

Add a comment pointing to a tcpdump-workers message where this is
mentioned.

dlpisubs.c

@@ -271,7 +271,16 @@ pcap_process_mactype(pcap_t *p, u_int mactype)
 
 #ifdef DL_IPNET
 	case DL_IPNET:
-		p->linktype = DLT_IPNET;
+		/*
+		 * XXX - DL_IPNET devices default to "raw IP" rather than
+		 * "IPNET header"; see
+		 *
+		 *    http://seclists.org/tcpdump/2009/q1/202
+		 *
+		 * We'd have to do DL_IOC_IPNET_INFO to enable getting
+		 * the IPNET header.
+		 */
+		p->linktype = DLT_RAW;
 		p->offset = 0;
 		break;
 #endif