Add script to detect filtered TCP traces, addresses BIT-1119.

If reading a trace file w/ only TCP control packets, a warning is
emitted to suggest the 'detect_filtered_traces' option if the user
doesn't desire Bro to report missing TCP segments for such a trace file.
Minor unified2 script documentation fix.
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  Minor unified2 script documentation fix.
Merge remote-tracking branch 'origin/topic/jsiwek/tcp-improvements'

* origin/topic/jsiwek/tcp-improvements:
  Add script to detect filtered TCP traces, addresses BIT-1119.

BIT-1119 #merged

If we could only disable an event handler dynamically ...