This makes conn.logs a bit prettier (and smaller) because all lines that
do not use a tunnel will now have a "-" instead of the "(empty)" for
tunnel_parents.
The extensions now work with optional types, as well with complex types
(like subrecords). Not returning a record in the ext_func no longer
crashes bro.
The default_ext_func was switched to return void in
cases where no extension revord is defined (was bool).
I also got rid of the offsets in the indices - with the rest of the
implementation, that was not really necessary and made the code more
complex.
