* implement deb822 suggestions from donkult (thanks!):

* implement deb822 suggestions from donkult (thanks!):
  - rename "Dist" to "Suites"
  - rename "Section" to "Sections"
  - rename "Architectures-Delete" to "Architectures-Remove"
  - rename "Uri" to "URI"
* add "apt list --manual-installed"
* add "apt upgrade --dist"
* add "apt purge"
* flock() the file edited in "apt edit-sources"
* apt-private/private-show.cc:
  - do not show Description-lang: header
* reword apt !isatty() warning
* add missing integration test for "apt list" and fix bugs
  found by it