dep_map duplicate removal script

[Joshua Root]

Joshua Root wrote:
> To fully resolve the issues with duplicate dependency map entries 
> reported in <http://trac.macports.org/ticket/8763>, I've attached a 
> small script that will remove existing duplicates. It should be run by 
> the postflight script in future MP binary packages, and on selfupdate 
> too if we can figure out a way of doing that.

I've now committed this script to trunk, and it will run during 'make
install'. Once the MacPorts port is updated so it adds the script to the
.dmg resources, it will be run in postflight too.

Testing appreciated. :-)

- Josh