Test ping with link-local addresses.

1. Create one PingReplyThread per netid.
2. Make PingReplyThread reply to link-local ping only to its
   router link-local address.
3. Ensure that ping to link-local addresses using various methods
   (scope ID, SO_BINDTODEVICE, IPV6_UNICAST_IF) gets a reply on
   all the netids.
4. Ensure that when SO_BINDTODEVICE is set, no other interface
   selection mechanism is permitted.

Change-Id: Ia15557bc6563a88b0d6504a5c95e5071f5c65d14
Bug: 29370996