collectd: sqm_collectd improve interface name filter

Increase the range of characters that get substituted by '_' so the
shell doesn't complain about illegal variable names.

Primarily done to catch '.' and '-' but who knows what funnies will
appear in i/f names.

It's a shame that busybox ash doesn't understand :alnum:

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

utils/collectd/files/exec-scripts/sqm_collectd.sh

@@ -9,7 +9,7 @@ handle_cake() {
 	local ifc ifr tin i
 
 	ifc="$1"
-	ifr="${ifc//./_}"
+	ifr="${ifc//[!0-9A-Za-z]/_}"
 
 	# Overall
 	json_get_vars bytes packets drops backlog qlen