GDateTime fixes to handle TZif footers

This series of changes fixes some bugs in timestamp handling caused because TZif files are not properly parsed by the GDateTime code. The bugs include issue #2129 (closed) along with some other bugs I noticed while in the neighborhood. The bugs have to do with features added to tzcode in 1995 and in 2013, and to POSIX in 2001. These features are standardized in Internet RFC 8536.

This is the first merge request I've ever submitted to GitLab so please bear with me. The first patch is "gtimezone: get 64-bit data from version-3 TZif files" and the last patch is "gtimezone: support footers in TZif files", but the UI seems to be listing these in reverse order. Hope that's OK.