MFC 230453, 230726, 252015, 252426:

MFC 230453, 230726, 252015, 252426:

- ipv6_enable + ipv6_gateway_enable should unset ACCEPT_RTADV by default for
  backward compatibility.

- Configurations in ipv6_prefix_IF should be recognized even if there is no
  ifconfig_IF_ipv6.

- DAD wait should be performed at once, not on a per-interface basis, if
  possible.  This fixes an issue that a system with a lot of IPv6-capable
  interfaces takes too long for booting.

- Add CIDR notation support like 192.168.1-2.10-16/24 to $ifconfig_IF_aliasN.
  This is an extended version of ipv4_addr_IF which supports both IPv4 and
  IPv6, and multiple range specifications.  To avoid to generate too many
  addresses, the maximum number of the generated addresses is currently
  limited to 31.

- Add $ifconfig_IF_aliases, which accepts multiple IP aliases in a variable.

- ipv6_prefix_IF now supports !/64 prefix length.  In addition to the old
  64-bit format (2001:db8:1:1), a full 128-bit format like 2001:db8:1:1::/64
  is supported.

- Replace ifconfig command with $IFCONFIG_CMD variable to support
  a dry-run mode in the future.

- Remove IP aliases before removing all of IPv4 addresses when doing
  "rc.d/netif down".

- Add a DAD wait to network6_getladdr() because it is possible to fail to
  configure an EUI64 address when ipv6_prefix_IF is specified.

git-svn-id: svn://svn.freebsd.org/base/stable/9@253227 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f