net: ipv6: allow choosing optimistic addresses with use_optimistic

The use_optimistic sysctl makes optimistic IPv6 addresses
equivalent to preferred addresses for source address selection
(e.g., when calling connect()), but it does not allow an
application to bind to optimistic addresses. This behaviour is
inconsistent - for example, it doesn't make sense for bind() to
an optimistic address fail with EADDRNOTAVAIL, but connect() to
choose that address outgoing address on the same socket.

Bug: 17769720
Bug: 18609055
Change-Id: I9de0d6c92ac45e29d28e318ac626c71806666f13
Signed-off-by: Erik Kline <ek@google.com>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: engstk <eng.stk@sapo.pt>