odhcp6c: preference and status code support in advertise

The patch implements handling of advertise messages
as described in RFC3315 paragraph 17.1.2. Client will
stop collecting advertise messages if preference option
is equal to 255 or if the first RT has elapsed. Status
codes are handled in the advertise message depending on
the startup parameters

Signed-off-by: Hans Dedecker <hans.dedecker@gmail.com>