alternative var=true joining

[jdx]

right now we just use shell_words::join() to deal with the problem of using var=true which populates $usage_foo as a space-delimited string where each arg may itself contain spaces.

I believe this logic just puts ' around each element if it has spaces. This is very bash-specific though, so it might not be desirable when using a different language. I'm open to alternatives here like customizing the escape behavior, or potentially having multiple env vars for this purpose like this:

usage_foo = "1 '2 3' 4"
usage_foo_n = "3"
usage_foo_0 = "1"
usage_foo_1 = "2 3"
usage_foo_2 = "4"

I didn't go with that initially because I think it would be a huge pain in bash, but the nice thing about it is it's much more formal about this and doesn't require any escaping.