fix more instances of ": $((a++))" in shell scripts

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

examples/mdev.conf.change_blockdev.sh

@@ -14,7 +14,7 @@ env | sort
 
 while sleep 1; test $cnt != 0; do
 	echo "Trying to reread partition table on $DEVNAME ($cnt)"
-	: $((cnt--))
+	cnt=$((cnt-1))
 	# If device node doesn't exist, it means the device was removed.
 	# Stop trying.
 	test -e "$DEVNAME" || { echo "$DEVNAME doesn't exist, aborting"; exit 1; }

examples/var_service/dhcp_if_pinger/run

@@ -30,7 +30,7 @@ test x"$router" != x"" || exec env - sleep "$ping_time"
 failcnt=0
 while true; do
 	ping $ping_opts "$router" && exec env - sleep "$ping_time"
-	: $((failcnt++))
+	failcnt=$((failcnt+1))
 	msg "Failed to ping $router, fail count:$failcnt"
 	test $failcnt -ge $max_fail && break
 	env - sleep "$retry_time"