Allow use of paths with spaces on Windows

[mattock]

The lack of quotes prevents placing easy-rsa into a directory which has spaces in its path, as reported by a user of ours.

These changes have not yet been tested at all, so consider this a discussion-only PR for now. Most of the changes should "just work", but there are a couple places where things could potentially break. I've added separate line comments in to these places.

Suggestions/improvements from people who have had to work with batch scripts are most welcome!