[th/nm-dhcp-config-merge]

For https://bugzilla.redhat.com/show_bug.cgi?id=1066703, NMDevice shall be reworked heavily.

As a first tiny step, merge the NMDhcp4Config and NMDhcp6Config implementations.

At the higher layer, DHCPv4 and DHCPv6 is sufficiently similar that code can be simplified by having a common API for both address families.