Re-add netd support for enabling IPv6 optimistic mode.

Enables optimistic_dad and use_optimistic on all interfaces
by default.

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This functionality was originally submitted in ag/569594.  Owing
to issues with bind()ing to optimistic addresses (see b/18609055)
this was reverted in ag/598690.

This reverts the revert.  :-)

Bug: 17769720
Change-Id: I156e5636341a846f41fb9a15c84cf06878b239d8