OS400: fix shellcheck warnings in scripts

- 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: #13307
Closes #13309

.github/scripts/shellcheck.sh

@@ -3,10 +3,8 @@
 #
 # SPDX-License-Identifier: curl
 
-# FIXME: packages/OS400/* scripts
-
 shellcheck --version
 # shellcheck disable=SC2046
 shellcheck --exclude=1091 \
   --enable=avoid-nullary-conditions,deprecate-which \
-  $(grep -l -E '^#!(/usr/bin/env bash|/bin/sh|/bin/bash)' $(git ls-files | grep -v -F 'packages/OS400/'))
+  $(grep -l -E '^#!(/usr/bin/env bash|/bin/sh|/bin/bash)' $(git ls-files))