sd-dhcp-client: make timeout handling a bit more robust

      Accept any lease lifetime greater than one second. Server should not
hand out extremely short leases, but let's not be the ones to fail.

Do not fail when arming a timer in the past, but also only arm one such
timer.

Avoid rounding errors when computing the default timeouts, this may be
an issue if we are handed a very short lease.

Also, don't pass 'time_now' around, as that can be found in the event
object when needed.