This replaces and extends the previous commit by using `$var` instead of
`$(var)` when this makes sense and using the ugly workaround for Doxygen
expanding environment variables even inside Markdown backticks elsewhere.
Also use backticks around more strings.
Use backticks around _UNICODE to avoid the special meaning of "_" in
Markdown.
Also use backticks around a couple of other identifiers/paths for
consistency.
