NMDhcpManager used a hash table to keep track of the dhcp client
instances. It never actually did a lookup of the client, the only
place where we search for an existing NMDhcpClient instance is
get_client_for_ifindex(), which just iterated over all clients.

Use a CList instead.

The only thing that one might consider a downside is that now
NMDhcpClient is aware of whether it is part of a list. Previously,
one could theoretically track a NMDhcpClient instance in multiple
NMDhcpManager instances. But that doesn't make sense, because
NMDhcpManager is a singleton. Even if we would have mulitple NMDhcpManager
instances, one client would still only be tracked by one manager.
This tighter coupling of NMDhcpClient and NMDhcpManager isn't
a problem.