Support parsing RDNSS ND options from netlink.

The RDNSS options (RFC 6106) used to configure DNS servers via
router advertisements are passed from the kernel to userspace via
RTM_NEWNDUSEROPT netlink messages. Add code to NetlinkEvent to
parse them.

Also fix a compiler warning and a couple of style issues.

Bug: 9180552
Change-Id: I6c532c8f0ceef3afdc977a431a036df398013e1a