sd-dhcp-client: refactor client_{free,new}

Make them more simiar to sd_bus and friends. Also factor out the event attachment. In the future,
we will likely want to support external main-loops, so this is a first step. For the time being,
we are still requiring an sd_event to be attached though.
networkd: dhcp - by default ignore the MTU
networkd: use 'up'/'down' rather than 'on'/'off'
bus-kernel: move bus_kernel_make_message

This makes future commits more readable.
sd-bus: let sd_bus_call() use the synchronous kdbus method