rework IPv6 dns address selection (FS#635)

rework IPv6 dns address selection (FS#635)

Don't return anymore the link local IPv6 address as DNS IPv6 address
since different OS implementations (e.g. android, ...) cannot handle
a link local IPv6 address as DNS address.

IPv6 DNS address selection is reworked as follows :
-Consider all global/ULA IPv6 address having a valid lifetime
-Give preference to global/ULA IPv6 addresses being not deprecated
-Give preference to ULA IPv6 addresses over IPv6 global addresses
-Give preference to the IPv6 address with the longest preferred lifetime in
its selected category (ULA or global)
-If no global/ULA IPv6 address is present use the IPv6 link local address

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>