Distinguish draft and release builds. (#364)

* Distinguish draft and release builds.

Rename the output PDFs so it's less likely to cause confusion.
The release build is not labelled as DRAFT, does not include the preface
warning, and does not include the change log.

* Add serial.tex, which got lost somewhere.

(Only used by future.tex.)

* Fix build.

Also add riscv-debug-release.tex which I forgot earlier.