dt(4): interval, profile: schedule clockintr relative to start of recording

dt(4): interval, profile: schedule clockintr relative to start of recording

For the interval and profile providers, schedule the first clock
interrupt to occur dp_nsecs nanoseconds after the start of recording.
This makes the interval between the start of recording and the first
event consistent across runs.

With input from claudio@.  Simplified by claudio@.

Thread: https://marc.info/?l=openbsd-tech&m=170879058205043&w=2

ok mpi@ claudio@