ip6-nd: stop sending RA by default

ip6-nd: stop sending RA by default

Type: improvement

Currently, RA message sending is enabled by default - both periodic and
in response to RS message. However, RFC 4861 section 6.2.1 says the
following:

  Note that AdvSendAdvertisements MUST be FALSE by default so that a
  node will not accidentally start acting as a router unless it is
  explicitly configured by system management to send Router
  Advertisements.

With this change, RA message sending is disabled by default and
"test_ip6.TestIPv6.test_rs" updated appropriately.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>
Change-Id: I2a8865199cb665c59268504aefe2976e5ee96dc2