netd: IPV6 tethering support

- Start NDP proxy daemon when tethering is enabled
- Add new interface commands "add_upstream" and "remove_upstream".
  Used to provide the complete set of interfaces to be proxied to
  the NDP proxy daemon
- Enable ipv6 forwarding
- After repeatedly toggling tethering/Wifi hotspot on and off, hotspot
  may not turn on anymore. It appears untethering is unsuccessful and
  this eventually leads to a timeout and a framework reboot. The NDP
  proxy daemon appears to be started multiple times even though there
  is no ipv6 upstream interface present. There appears to be a signifcant
  number of calls to fork and exec this daemon even though this
  functionality is not needed in this case. Fix this overhead by invoking
  the daemon only if there are atleast 2 interfaces to establish a bridge
  and proxy the NDP messages.

Conflicts:
	server/TetherController.cpp

CRs-fixed: 665103
Change-Id: I89170f8acd60b949ede749f353acd27363ff5798