"extract" script for extraction. Many cases where a custom EXTRACT_CMD
simply copied the distfile into the work directory are no longer
needed. The extract script also hides differences between pax and
tar behind a common command-line interface, so we no longer need code
that's conditional on whether EXTRACT_USING is tar or pax.
MD5 Collision Generation
Faster implementation of techniques in How to Break MD5 and Other Hash
Functions, by Xiaoyun Wang, et al.
Old (Wang, et al.) average run time on IBM P690 supercomputer - 1 hour
New average run time on P4 1.6ghz PC - 45 minutes