dep_map duplicate removal script

[Joshua Root]

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'm not sure where it should live though, both in the source tree and 
when installed. Any suggestions?

- Josh