sd-dhcp-client: drop logging about T1/T2

These are redundant now that the REQUEST messages contain the same information.
networkd: wait for IFF_RUNNING rather than IFF_LOWER_UP

The interface is not fully ready until it enterns RUNNING. This was causing
problems with sending out DHCP messages before the interface was ready, so they
would get lost. In particular this affected DHCP INIT-REBOOT, as it relies on
the first package sent being successful (or it will fall back to a full reboot).

Also improve the logging a lot, to make future debugging of link state a lot
easier.