Add to some comments notes on the meaning of DLT types 15 and 16 on

Linux systems with the isdn4linux patches; they help make DLT types even
less useful than they were after the various flavors of BSD proceeded to
add their own types past 14, with no coordination whatosever, so that
they overlapped, rendering it impossible to read a libpcap capture file
without knowing what particular OS generated it.

svn path=/trunk/; revision=1442

wiretap/libpcap.c

@@ -1,6 +1,6 @@
 /* libpcap.c
  *
- * $Id: libpcap.c,v 1.28 1999/12/15 02:25:50 guy Exp $
+ * $Id: libpcap.c,v 1.29 2000/01/09 07:55:48 guy Exp $
  *
  * Wiretap Library
  * Copyright (c) 1998 by Gilbert Ramirez <gram@verdict.uthscsa.edu>
@@ -139,10 +139,22 @@ static const int pcap_encap[] = {
 				   on OpenBSD and BSD/OS, DLT_RAW */
 	WTAP_ENCAP_UNKNOWN,	/* In OpenBSD and BSD/OS, BSD/OS SLIP,
 				   but the BSD/OS header says "internal
-				   to libpcap", whatever that means */
+				   to libpcap", whatever that means;
+				   in Linux with the ISDN4Linux patches
+				   applied to libpcap, DLT_I4L_RAWIP,
+				   which looks just like DLT_RAW but
+				   is given a different DLT_ code for
+				   no obvious good reason */
 	WTAP_ENCAP_UNKNOWN,	/* In OpenBSD and BSD/OS, BSD/OS PPP,
 				   but the BSD/OS header says "internal
-				   to libpcap", whatever that means */
+				   to libpcap", whatever that means;
+				   in Linux with the ISDN4Linux patches
+				   applied to libpcap, DLT_I4L_IP,
+				   which provides only a 2-octet
+				   Ethernet type as a link-layer header,
+				   with a type of 0xFFFF meaning
+				   ETH_P_802_3, a "Dummy type for 802.3
+				   frames" */
 	WTAP_ENCAP_UNKNOWN,
 	WTAP_ENCAP_UNKNOWN,
 	WTAP_ENCAP_LINUX_ATM_CLIP