Some more improvements for the packaging/release-rsync script:

- Check early if the version tag already exists, so it aborts right
  away if the release script can't do its work.
- Update the files in the "patches" dir while merging the master branch
  into the patch branches (done before creating the release patches for
  the rsync-patches tar file).
- Allow the user to ask to visit each patch when updating them.
- Pause after initial patch updating so that any extra patch changes
  can be done before the creating of the tar files.
- Ask for the GPG signing passphrase once for all signing commands.