Update to Argus release 2.0.6.
The clients are no longer part of this port, but will be built by
the argus-clients port, which is listed as a dependency.
Since both the argus and argus-clients ports contain the same
COPYING file and want to install it into the same directory, I have
disabled installation of the copyright file by this port, assuming
that the dependent argus-clients port already did it ...