Fix all of the include guards for all of the headers.

Also fix copyright statements in headers.
Technically, each person's contributions are equivalent from a licensing perspective, so I can add terms at the top as long as they don't conflict with the existing terms. In any case, none of the contributors are likely to come back and say "I contributed to a GPL v2 project but I didn't mean to contribute under the GPL v2." That'd just be silly. :/ Though, IANAL as well...