odhcp6c: add option "keep_ra_dnslifetime"

Add option keep_ra_dnslifetime which will preserve the received
lifetime for RDNSS and DNSSL RA records and not overwrite it
by the RA router lifetime as specified in RFC6106.
This allows to accept RDNNS records from RAs that don't announce
a default route by setting router lifetime to 0 in the RAs.

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