zdiff: avoid non-conformant features

[kevans]

Summary

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.

Sponsored by: Klara, Inc.

[jilles]

usr.bin/gzip/zdiff

41–42

This used to work with filenames containing ", ` and $ but no longer does. Likewise for all other eval "$2= below.

[kevans]

Fix files containing ", `, and $. Add tests to cover those names, as well, for
all of these cases.