ipv6: Do not assign non-valid address on interface.

Check the type of the address when adding a new one on interface.
- the unspecified address (::) is always disallowed (RFC4291 2.5.2)
- the loopback address is disallowed unless the interface is (one of)
  loopback (RFC4291 2.5.3).
- multicast addresses are disallowed.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>