Split $ipv6_prefer into $ip6addrctl_policy and $ipv6_activate_all_interfaces.

Split $ipv6_prefer into $ip6addrctl_policy and $ipv6_activate_all_interfaces.

The $ip6addrctl_policy is a variable to choose a pre-defined address
selection policy set by ip6addrctl(8).
The keyword "ipv4_prefer" sets IPv4-preferred one described in Section 10.3,
the keyword "ipv6_prefer" sets IPv6-preferred one in Section 2.1 in RFC 3484,
respectively.  When "AUTO" is specified, it attempts to read
/etc/ip6addrctl.conf first.  If it is found, it reads and installs it as
a policy table.  If not, either of the two pre-defined policy tables is
chosen automatically according to $ipv6_activate_all_interfaces.

When $ipv6_activate_all_interfaces=NO, interfaces which have no corresponding
$ifconfig_IF_ipv6 is marked as IFDISABLED for security reason.

The default values are ip6addrctl_policy=AUTO and
ipv6_activate_all_interfaces=NO.

Discussed with:	ume and bz