Patch by Jim Ahlstrom to add crc32, a useful checksum function

Patch by Jim Ahlstrom to add crc32, a useful checksum function
(e.g. used for ZIP files).

The patch includes code that says:
+  Copyright (C) 1986 Gary S. Brown.  You may use this program, or
+  code or tables extracted from it, as desired without restriction.

My interpretation (and Jim's) is that Gary S Brown has no claims under
copyright, patent or other rights or interests.  Lawyers might disagree.