Consider an alternative item separator for multi-item option values

Javadoc options, such as `noqualifier`, `exclude`, `tag`, etc., use `:` (colon) to separate items in their multi-item value. For example:

    -exclude java.net:java.lang:java.io

Unix uses `:` to separate file paths. For example:

```
-docletpath dir1:dir2\subdir2
```

While the character is the same, the separators it represents are semantically different. The path separator changes with the platform, whereas the value separator does not. These superficial similarity of separators on Unix might trick users into thinking that they must change both when on another platform.

We could define an alternative value separator that does not clash with the path separator on any of the supported platforms. For example, `,` (comma). We could also define the value separator once (in the man page and elsewhere) and refer to it in every affected option. We just have to make sure that an individual item cannot contain `,` for every multi-item option value.