ip: add vxcan/veth to ip-link man page

ip: add vxcan/veth to ip-link man page

veth and vxcan both create a vitual tunnel between a pair of virtual network
devices. This patch adds the content for the now supported vxcan netdevices
and the documentation to create peer devices for vxcan and veth.

Additional remove 'can' that accidently was on the list of link types which
can be created by 'ip link add' as 'can' devices are real network devices.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>