Copied DELETE-DIR from Rebol 2 (cc#1545)

Since there was no strong opinions on this
in the http://issue.cc/r3/1545 ticket
I would keep it in Rebol 3 to converge towards
the level of usefulness Rebol 2 provided.
Merge pull request #137 from onetom/delete-dir

Copied DELETE-DIR from Rebol 2 (cc#1545)