Add "Automatic (DHCP only)" as possible IPv6 activation method. The

default IPv6 activation method is "Automatic" now.
IPv6 supports "Automatic", "Automatic (addresses only)" and
"Automatic (DHCP only)". IPv4 supports "Automatic (DHCP)" and "Automatic
(DHCP) addresses only".

BUG: 297424
FIXED-IN: 0.9.0.3
(cherry picked from commit 6c30f58a)