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>

sh/rc-mount.sh

@@ -59,7 +59,7 @@ do_unmount()
 						eend 1
 					else
 						local sig="TERM"
-						retry=$(($retry - 1))
+						: $(( retry -= 1 ))
 						[ $retry = 1 ] && sig="KILL"
 						fuser $f_kill$sig -k $f_opts \
 							"$mnt" >/dev/null 2>&1