dhcp: merge nm_dhcp_client_start_ip4() and nm_dhcp_client_start_ip6() implementations

As almost always, there is a point in keeping IPv4 and IPv6 implementations
similar. Behave different where there is an actual difference, at the bottom
of the stack.
57 jobs for th/dhcp-cleanup-1 in 35 minutes and 56 seconds (queued for 4 seconds)