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

salt/sys-net/files/server/rpc/qusal.ConnectTCP

@@ -17,6 +17,7 @@
 
 set -eu
 
+# shellcheck disable=SC2154
 arg="${QREXEC_SERVICE_ARGUMENT}"
 host="${arg%%+*}"
 port="${arg##*+}"