feat: enable all optional shellcheck validations

Make shell a little bit safer with:

- add-default-case
- check-extra-masked-returns
- check-set-e-suppressed
- quote-safe-variables
- check-unassigned-uppercase

Although there are some stylistic decisions for uniformity:

- avoid-nullary-conditions
- deprecated-which
- require-variable-braces

scripts/salt-fix.sh

@@ -13,19 +13,24 @@
 set -eu
 
 command -v git >/dev/null || { echo "Missing program: git" >&2; exit 1; }
-cd "$(git rev-parse --show-toplevel)" || exit 1
+repo_toplevel="$(git rev-parse --show-toplevel)"
+test -d "${repo_toplevel}" || exit 1
+unset repo_toplevel
 
 find_tool="$(./scripts/best-program.sh fd fdfind find)"
 
 case "${find_tool}" in
   fd|fdfind)
-    files="$(${find_tool} . minion.d/ --extension=conf)
-      $(${find_tool} . salt/ --max-depth=2 --type=f --extension=sls)"
+    conf_files="$(${find_tool} . minion.d/ -e conf)"
+    sls_files="$(${find_tool} . salt/ -d 2 -t f -e sls)"
+    files="${conf_files}\n${sls_files}"
     ;;
   find)
-    files="$(find minion.d/ -type f -name "*.conf")
-      $(find salt/ -maxdepth 2 -type f -name '*.sls')"
+    conf_files="$(find minion.d/ -type f -name "*.conf")"
+    sls_files="$(find salt/ -maxdepth 2 -type f -name '*.sls')"
+    files="${conf_files}\n${sls_files}"
     ;;
+  *) echo "Unsupported find tool" >&2; exit 1;;
 esac
 
 ## 201 - Fix trailing whitespace: