change ports default work directory prefix

[Dirk Engling]

Today in EuroBSDCon's jail working group we discussed changing the
default for WRKDIRPREFIX to /usr/obj/ports. This has the advantage of
being able to share the ports tree between host system and jails.
Another plus is that cleaning all work directories is much faster than a
recursive make clean.

With the current default, exposing the ports tree to jails potentially
leaks information about installed programs, configured options or host
specific generated secrets (thinking of LocalSettings.php).

On the down side, developers can't by default just copy the port, hack
away and be sure to only modify files in their respective home directories.

bapt@ asked me to discuss this here, also looking for potential other
pitfalls I have not thought about.

Regards,

  erdgeist