This change allows to exec some cmd on each
named netns (except default) by specifying '-all' option:

    # ip -all netns exec ip link

Each command executes synchronously.

Exit status is not considered, so there might be a case
that some CMD can fail on some netns but success on the other.

EXAMPLES:

1) Show link info on all netns:

$ ip -all netns exec ip link

netns: test_net
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
4: tap0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 500
    link/ether 1a:19:6f:25:eb:85 brd ff:ff:ff:ff:ff:ff

netns: home0
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
4: tap0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 500
    link/ether ea:1a:59:40:d3:29 brd ff:ff:ff:ff:ff:ff

netns: lan0
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
4: tap0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 500
    link/ether ce:49:d5:46:81:ea brd ff:ff:ff:ff:ff:ff

2) Set UP tap0 device for the all netns:

$ ip -all netns exec ip link set dev tap0 up

netns: test_net

netns: home0

netns: lan0
Signed-off-by: Vadim Kochan <vadim4j@gmail.com>