Also:
- merge CI check and shellcheck jobs into a single one.
To share the same shellcheck version and less overhead.
- use `set -eu` in more scripts.
- make sure CI scripts run from any cwd.
To make it easy to run them on local machine.
- minor tidy-ups.
Closes#1618
- use `$()` instead of backticks, and re-arrange double-quotes inside.
- add missing `|| exit 1` to `cd` calls. (could be dropped by using `set -eu`.)
- add `-n` to a few `if`s.
- shorten redirections by using `{} >` (as shellcheck recommended).
- silence warnings where variables were detected as unused (SC2034).
- a couple misc updates to silence warnings.
- switch to bash shebang for `-ot` feature.
- split two lines to unbreak syntax highlighting in my editor. (`$(expr \`, `$(dirname \`)
Also enable CI checks for OS/400 shell scripts.
Ref: d88b9bcdafCloses#1358
