ip: set the close-on-exec flag for descriptors

ip: set the close-on-exec flag for descriptors

Otherwise a program executed by "ip netns exec" has two extra
descriptors.

$ ip netns exec test /bin/bash
$ lsof -p $$
...
bash    817 root    0u   CHR  136,0       0t0          3 /dev/pts/0
bash    817 root    1u   CHR  136,0       0t0          3 /dev/pts/0
bash    817 root    2u   CHR  136,0       0t0          3 /dev/pts/0
bash    817 root    3u  sock    0,6       0t0      13386 protocol: NETLINK
bash    817 root    4r   REG    0,3         0 4026532155 net
bash    817 root  255u   CHR  136,0       0t0          3 /dev/pts/0

Cc: Stephen Hemminger <stephen@networkplumber.org>
Reported-by: Dilip Daya <dilip.daya@hp.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>