dhcpcd service: fix network-online.target integration

When dhcpcd instead of networkd is used, the network-online.target behaved
the same as network.target, resulting in broken services that need a working
network connectivity when being started.

This commit makes dhcpcd wait for a lease and makes it wanted by
network-online.target. In turn, network-online.target is now wanted by
multi-user.target, so it will be activated at every boot.