Add missing GPL headers, and copyright claims that certainly apply.

Add missing GPL headers, and copyright claims that certainly apply.

* Fix (a subset of)? files with non-trivial code that are missing GPL headers.

* A few copyright claims added which I am certain apply, but which I had
  missed out on the original commits.

NB: Copyright claims are not exclusive and the addition of any copyright
claim should not be read as implying a lack of any further claims, or
denying the validity of any other claims.  All those with claims of
copyright over any portion of Quagga are welcome to submit them, ideally as
patches to update copyright strings in files.

(cherry picked from commit 010ebbbca6396f272cc2d50d147dd922dda68213)