Improvements and fixes to zsh completion

* Use correct return status so that approximate completion isn't broken
* Follow zsh conventions on description forms
* Some options can be used multiple times, e.g. -H, -r, --file-name
* Set completion context correctly for the cache subcommand
* Better completion for --map-syntax argument
* Add --nonprintable-notation option
* Correct some of the mutual exclusion lists for options

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Refactor and cleanup build script #2756 (@cyqsimon)
 - Checks changelog has been written to for PRs in CI #2766 (@cyqsimon)
 - Minor benchmark script improvements #2768 (@cyqsimon)
+- Update and improve `zsh` completion, see #2772 (@okapia)
 
 ## Syntaxes