Remove libnl++ dependency on NETLINK_ROUTE.

One exception is for RTA_* definitions at NetlinkRequest. This class is
going to be refactored anyway, so let's focus on this later.

While we're here, also replace type assumptions with an actual
decltype lookup.

Bug: 162032964
Test: canhalctrl up test virtual vcan123
Change-Id: I3b509fa7b1870d4de7302fb5221a06613dfbb817