'find-dups' was trying to call `mu remove' for each duplicate message it
removes. This can be quit slow, so simply delete a file. After dups have
been removed, run `mu index'.
Closes#592
Uses open-pipe* to ensure that escaping isn't needed (seen in md5sum)
since arguments are passed.
Also see this discussion: http://thread.gmane.org/gmane.lisp.guile.user/11777
Experimental feature... be careful.
