zdiff: avoid non-conformant features

`setvar` is a non-conformant feature that looks slightly neater but is
not portable to other /bin/sh implementations. Making the script
portable is straightforward, so let's do it.

Tests are added to make sure that I didn't break anything major in the
process.

Reviewed by: bapt (previous version), jilles
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35275