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/ifplugd.sh

@@ -72,7 +72,7 @@ ifplugd_pre_start()
 		fi
 		sleep 1
 		[ ${timeout} -eq 0 ] && continue
-		i=$((${i} + 1))
+		: $(( i += 1 ))
 		[ ${i} -ge ${timeout} ] && break
 	done