Remove the netlink code to get the tun if index.

It only worked for kernel >= 2.6.33. Use if_nametoindex() instead, which made
me include net/if.h in place of linux/if.h. Also, fix the RTM_SETLINK use which
should really be RTM_NEWLINK with correct parameters.

Signed-off-by: Benjamin Cama <benoar@dolka.fr>