[type/__postgres_conf] Add psql_conf_cmp function to state explorer

cdist/conf/type/__postgres_conf/explorer/state

@@ -27,12 +27,16 @@ psql_exec() {
 	su - "${postgres_user}" -c "psql postgres -twAc $(quote "$*")"
 }
 
+psql_conf_cmp() {
+	test "$(psql_exec "SHOW $1")" = "$2"
+}
+
 psql_exec 'SELECT 1' >/dev/null || {
 	echo 'Connection to PostgreSQL server failed' >&2
 	exit 1
 }
 
-if psql_exec "SHOW ${conf_name}" | cmp -s "${__object:?}/parameter/value" -
+if psql_conf_cmp "${conf_name}" "$(cat "${__object:?}/parameter/value")"
 then
 	echo present
 else