get-pup: Verify with openssl instead of sha256sum

[ryandesign]

Use openssl to verify download because sha256sum does not exist on macOS.

If there are systems that have sha256sum but not openssl, then this could be rewritten to use whichever of those is present.

[aperezdc]

This is actually a good idea, we can expect OpenSSL will be installed because it's a dependency of Nginx itself, needed anyway in order to build and run tests. Thanks for the patch, @ryandesign 👌🏼

[ryandesign]

Well nginx can be built without ssl support, and the version you build for tests is built that way. Still, openssl is very popular and is probably installed.

Of course, this PR doesn't solve the problem that the pup binary that gets downloaded for macOS just crashes immediately and has to be rebuilt from source (by downloading and extracting the pup source and running go build in its directory), but one thing at a time. 😃