Previously, a single `icmp_conn` record was built per ICMP "connection"
and re-used for all events generated from it. This may have been a
historical attempt at performance optimization, but:
* By default, Zeek does not load any scripts that handle ICMP events.
* The one script Zeek ships with that does handle ICMP events,
"detect-traceroute", is already noted as being disabled due to
potential performance problems of doing that kind of analysis.
* Re-use of the original `icmp_conn` record tends to misreport
TTL and length values since they come from original packet instead
of the current one.
* Even if we chose to still re-use `icmp_conn` records and just fill
in a new TTL and length value each packet, a user script could have
stored a reference to the record and not be expecting those values
to be changed out from underneath them.
Now, a new `icmp_info` record is created/populated in all ICMP events
and should be used instead of `icmp_conn`. It also removes the
orig_h/resp_h fields as those are redundant with what's already
available in the connection record.
* All "Broxygen" usages have been replaced in
code, documentation, filenames, etc.
* Sphinx roles/directives like ":bro:see" are now ":zeek:see"
* The "--broxygen" command-line option is now "--zeexygen"
