MFC r226649, 226651, 226652, 226653:

MFC r226649, 226651, 226652, 226653:
- Fix an issue that 127/8 is not configured when $ifconfig_DEFAULT is not empty.
- Add description that IPv6 configuration will be ignored if $ifconfig_IF_ipv6
  is empty.
- Move a configuration example "inet6 accept_rtadv" to just after the manual
  GUA configuration.
- Add an example of $ipv6_prefix_IF.
- Add support for removing addresses added by ipv6_prefix_hostid_addr_up()
  upon rc.d/netif stop.

Approved by:	re (bz)

git-svn-id: svn://svn.freebsd.org/base/stable/9@228245 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f