make shell math operations style more succulent

Convert the style:
	var=$((${var} + 1))
to:
	: $(( var += 1 ))

The latter is easier to read imo.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

support/openvpn/up.sh

@@ -34,7 +34,7 @@ while true; do
 	elif [ "${opt}" != "${opt#dhcp-option DNS *}" ]; then
 		NS="${NS}nameserver ${opt#dhcp-option DNS *}\n"
 	fi
-	i=$((${i} + 1))
+	: $(( i += 1 ))
 done
 
 if [ -n "${NS}" ]; then