router: use ra_lifetime as lifetime for RA options (FS#2206)

router: use ra_lifetime as lifetime for RA options (FS#2206)

Use the RA lifetime as lifetime for the recursive DNS and DNS search options
For the route options use as lifetime the smallest value of either the valid
lifetime of the address associated with the route or the RA lifetime.
This avoids routes having infinite lifetime being installed in downstream
clients which can results into routes pointing to a removed router.

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