Optimally, NMDhcpClient would be stateless and all paramters would
be passed on as argument. Clearly that is not feasable, because there
are so many paramters, and in many cases they need to be cached for the
lifetime of the client instance.

Instead of passing info_only paramter to ip6_start() and cache it
both in NMDhcpClient and NMDhcpSystemd, keep it in NMDhcpClient at
one place.

In the next commit, we will initialize info-only only once during the
constructor, so it is immutable and somewhat stateless.