ipv6: addrconf: implement address generation modes

This patch introduces a possibility for userspace to set various (so far
two) modes of generating addresses. This is useful for example for
NetworkManager because it can set the mode to NONE and take care of link
local addresses itself. That allow it to have the interface up,
monitoring carrier but still don't have any addresses on it.

One more use-case by Dan Williams:
<quote>
WWAN devices often have their LL address provided by the firmware of the
device, which sometimes refuses to respond to incorrect LL addresses
when doing DHCPv6 or IPv6 ND.  The kernel cannot generate the correct LL
address for two reasons:

1) WWAN pseudo-ethernet interfaces often construct a fake MAC address,
or read a meaningless MAC address from the firmware.  Thus the EUI64 and
the IPv6LL address the kernel assigns will be wrong.  The real LL
address is often retrieved from the firmware with AT or proprietary
commands.

2) WWAN PPP interfaces receive their LL address from IPV6CP, not from
kernel assignments.  Only after IPV6CP has completed do we know the LL
address of the PPP interface and its peer.  But the kernel has already
assigned an incorrect LL address to the interface.

So being able to suppress the kernel LL address generation and assign
the one retrieved from the firmware is less complicated and more robust.
</quote>

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>