Replace GENERATOR and VERSION with THISREV

Instead of replacing existing `<html:meta name="generator">`s, format
them into a comma‐separated list with ⛩️📰 书社 as the final entry.
Don’t allow overriding of ⛩️📰 书社 generator metadata.

Manually specifying `THISREV` is still possible to allow it to be
filled by users running Make without Git, but it should not be
overridden with the version of the calling generator, as it is used to
fill `@书社:version`.