treewide: improve template expansion

also: support *.toml as extra configuration dictionaries

scripts/envsubst-multi

@@ -0,0 +1,20 @@
+#!/bin/sh
+set -ef
+
+__have_args="${ENVSUBST_ARGS:+1}"
+if [ -z "${__have_args}" ] ; then
+	## optimize envsubst-single invocation by caching argument list
+	## ref: envsubst-single
+	ENVSUBST_ARGS=$(mktemp)
+	envsubst-args.sh > "${ENVSUBST_ARGS}"
+	export ENVSUBST_ARGS
+fi
+
+set +e ; __ret=0
+for i ; do
+	envsubst-single "$i" || __ret=1
+done
+
+[ -n "${__have_args}" ] || rm -f "${ENVSUBST_ARGS}"
+
+exit ${__ret}