If '-nm' specified that do not fail if there is no
default class names file in /etc/iproute2.

Changed default class name file cls_names -> tc_cls.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

It is possible to use class names from file /etc/iproute2/cls_names
which tc will use when showing class info:

    # tc/tc -nm class show dev lo
	class htb 1:10 parent 1:1 leaf 10: prio 0 rate 5Mbit ceil 5Mbit burst 15Kb cburst 1600b
	class htb 1:1 root rate 6Mbit ceil 6Mbit burst 15Kb cburst 1599b
	class htb web#1:20 parent 1:1 leaf 20: prio 0 rate 3Mbit ceil 6Mbit burst 15Kb cburst 1599b
	class htb 1:2 root rate 6Mbit ceil 6Mbit burst 15Kb cburst 1599b
	class htb 1:30 parent 1:1 leaf 30: prio 0 rate 1Kbit ceil 6Mbit burst 15Kb cburst 1599b
	class htb voip#1:40 parent 1:2 leaf 40: prio 0 rate 5Mbit ceil 5Mbit burst 15Kb cburst 1600b
	class htb 1:50 parent 1:2 leaf 50: prio 0 rate 3Mbit ceil 6Mbit burst 15Kb cburst 1599b
	class htb 1:60 parent 1:2 leaf 60: prio 0 rate 1Kbit ceil 6Mbit burst 15Kb cburst 1599b

or to specify via file path:

    # tc/tc -nm -cf /tmp/cls_names class show dev lo

Class names file contains simple "maj:min  name" structure:

1:20    web
1:40    voip

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>