LWAPP: Modernize packet parsing style.

The first revision of print-lwapp.c supposedly implemented
draft-ohara-capwap-lwapp-04, all subsequent revisions made various code
clean-ups only and the differences between the I-D and RFC 5412 seem to
be purely editorial.  However, the current implementation has
substantial discrepancies with the specification.  Moreover, several
aspects of the specification are plainly not implementable due to
various technical issues.  The matter is, RFC 5412 was published for
posterity rather than implementation: CAPWAP superseded LWAPP before the
latter was complete.

Before time is good for a more substantial reorganization of tcpdump
decoders, clean the LWAPP decoder up once more and make it follow the
specification better where practicable.  Improve various comments.
Define ND_LONGJMP_FROM_TCHECK.

In lwapp_transport_header rename version to ver_rid_cfl and rearrange
the associated macros for clarity.

lwapp_control_print() and lwapp_data_print() each parse an LWAPP
transport header in a slightly different way.  Move the common code to a
new function, lwapp_print(), and make it the only function exported from
the decoder.  To print the value of "length", the old common code used
UDP payload length for ndo_vflag == 0 and LWAPP transport header payload
length otherwise; in the new function make it the latter in both cases.
Ibid., add diagnostics to flag unexpected fragmentation in the transport
header.  Ibid., use the transport header C bit instead of UDP ports to
tell control and data frames apart.

Rename lwapp_message_header to lwapp_msgelem_header to match the data it
represents.  Add a registry of message elements and specify the minimum
length where possible.  Implement a new function, permitted_msg_elem(),
to tell whether a message element is valid for a message type.  Fix
lwapp_control_print() to iterate over message elements rather than
messages and use the new props to validate and to print control messages
a bit better.

In lwapp_data_print() remove the mention of AP identity field because
the function does not actually implement it.  Ibid., do not require the
transport payload length to be greater or equal to the transport header
length and do not subtract the latter from the former for the hex dump
because the former does not include the latter (this recovers the
overlooked last 6 bytes of the payload in the output); require the
transport payload end and the UDP payload end to be the same.  With
these changes made lwapp_data_print() becomes equivalent to just
print_unknown_data(), so remove the former function.

Since these clean-ups affect most of the code, in the same go reindent
the entire file in a more sensible way.  Update the tests.

Author: infrastation

netdissect.h

@@ -710,8 +710,7 @@ extern void lldp_print(netdissect_options *, const u_char *, u_int);
 extern void lmp_print(netdissect_options *, const u_char *, u_int);
 extern void loopback_print(netdissect_options *, const u_char *, u_int);
 extern void lspping_print(netdissect_options *, const u_char *, u_int);
-extern void lwapp_control_print(netdissect_options *, const u_char *, u_int, int);
-extern void lwapp_data_print(netdissect_options *, const u_char *, u_int);
+extern void lwapp_print(netdissect_options *, const u_char *, const u_int, const u_char);
 extern void lwres_print(netdissect_options *, const u_char *, u_int);
 extern void m3ua_print(netdissect_options *, const u_char *, const u_int);
 extern int macsec_print(netdissect_options *, const u_char **, u_int *, u_int *, u_int *, const struct lladdr_info *, const struct lladdr_info *);