Add option to ignore default lifetime for RDNSS records

While RFC6106 mandates that the RDNSS lifetime is capped to the default
lifetime, this behaviour is often undesirable. In particular, it prevents
accepting RDNSS records from RAs that don't also advertise a default route
(set the default lifetime to 0).

Therefore, make it possible to opt out of this behaviour and respect the
RDNSS lifetime independently of the default lifetime using the new command
line switch -L.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>