sd-event: don't mistake USEC_INFINITY passed in for overflow

sd-event: don't mistake USEC_INFINITY passed in for overflow

Let's pass USEC_INFINITY from sd_event_source_set_time_relative() to
sd_event_source_set_time() instead of raising EOVERFLOW.

We should raise EOVERFLOW only if your addition fails, but not if the
input already is USEC_INFINITY, since it's an entirely valid operation
to have an infinite time-out, and we should support that.