Merge from vendor branch TNF:

Merge from vendor branch TNF:
Import zidrav from pkgsrc-wip.  Packaged by Tobias Nygren.

Zidrav is a file corruption detection and repair program.  It is
designed to provide a way to fix file damage without having to
redownload the entire (potentially large) file over a (potentially
slow) connection.  A checksum file is generated for the broken file.
The checksum file can then be used together with the correct file to
produce a binary patch file for repair.