Fixes for IPv6 truncation and ICMP/ICMP6 analysis.

- Add more guards against trying to analyze captured packets with a
  truncated IPv6 static header or extension header chain.

- Add back in the ICMP payload tracking for ICMP "connections".

- Fix 'icmp_context' record construction.  Some field assignments
  were mismatched for ICMP and ICMP6.  Source and destination
  addresses were set incorrectly for context packets that don't
  contain a full IP header.  Some fields for ICMP6 weren't filled out.

- Changed ICMP Time Exceeded packets to raise the 'icmp_time_exceeded'
  event instead of 'icmp_error_message'.

- Add unit tests for truncation and the main types of ICMP/ICMP6
  that have specific events.

- Documentation clarifications.

scripts/base/init-bare.bro

@@ -102,11 +102,13 @@ type icmp_conn: record {
 ## .. bro:see:: icmp_time_exceeded icmp_unreachable
 type icmp_context: record {
 	id: conn_id;	##< The packet's 4-tuple.
-	len: count;	##< The lenght of the packet's IP header.
+	len: count;	##< The length of the IP packet (headers + payload).
 	proto: count;	##< The packet's transport-layer protocol.
 	frag_offset: count;	##< The packet's fragementation offset.
-	## True if the packet's IP header is fully included in the context. If that is not
-	## the case, the other fields will all be set to null values.
+	## True if the packet's IP header is not fully included in the context
+	## or if there is not enough of the transport header to determine source
+	## and destination ports. If that is the cast, the appropriate fields
+	## of this record will be set to null values.
 	bad_hdr_len: bool;
 	bad_checksum: bool;	##< True if the packet's IP checksum is not correct.
 	MF: bool;	##< True if the packets *more fragements* flag is set.