netd: IPv6 Tethering Support

This patch modifies netd to receive addUpstreamInterface and
removeUpstreamInterface commands from the tethering java
framework. Adds v6start/v6stop which calls the ICMPv6 forwarder
with the appropriate interfaces. Enables IPv6 forwarding.

Change-Id: Ic857f15ea9c5ec4be0dbde2656b55e65060f4497