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>

net/pppd.sh

@@ -96,7 +96,7 @@ pppd_pre_start()
 	if ! ${hasdefaultmetric}; then
 		local m=
 		eval m=\$metric_${IFVAR}
-		[ -z "${m}" ] && m=$((${metric} + $(_ifindex)))
+		[ -z "${m}" ] && : $(( m = metric + $(_ifindex) ))
 		opts="${opts} defaultmetric ${m}"
 	fi
 	if [ -n "${mtu}" ]; then