Making a new tagged Jumbo release

1. Ensure .gitattributes file is up to date.
2. First do a final update of CHANGES-jumbo and CREDITS-jumbo!
3. Define (de-comment) JTR_RELEASE_BUILD in params.h and Makefile.legacy
4. Commit that and tag that commit as eg. 1.9.0-Jumbo-1 (NB, git push --tags)
5. Undo §3 in next commit, going back to bleeding...