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: d88b9bcdaf
Closes #1358

os400/make-include.sh

@@ -1,13 +1,13 @@
-#!/bin/sh
+#!/usr/bin/env bash
 # Copyright (C) The libssh2 project and its contributors.
 # SPDX-License-Identifier: BSD-3-Clause
 #
 #       Installation of the header files in the OS/400 library.
 #
 
-SCRIPTDIR=`dirname "${0}"`
+SCRIPTDIR=$(dirname "${0}")
 . "${SCRIPTDIR}/initscript.sh"
-cd "${TOPDIR}/include"
+cd "${TOPDIR}/include" || exit 1
 
 
 #       Create the OS/400 source program file for the header files.
@@ -47,11 +47,11 @@ copy_hfile()
 #       Copy the header files.
 
 for HFILE in *.h "${TOPDIR}/os400/libssh2_ccsid.h"
-do      DEST="${SRCPF}/`db2_name \"${HFILE}\"`.MBR"
+do      DEST="${SRCPF}/$(db2_name "${HFILE}").MBR"
 
         if action_needed "${DEST}" "${HFILE}"
         then    copy_hfile "${DEST}" "${HFILE}"
-                IFSDEST="${IFSINCLUDE}/`basename \"${HFILE}\"`"
+                IFSDEST="${IFSINCLUDE}/$(basename "${HFILE}")"
                 rm -f "${IFSDEST}"
                 ln -s "${DEST}" "${IFSDEST}"
         fi