Because et_start() uses the delta between timestamps to compute the next event,
it is important to get the current sbinuptime() just before calling et_start(),
else the time spent to execute code and obtaining mutexes, since the last
sbinuptime() call gets accumulated as delay for the next timer event.
Tested using constant 1ms timers firing from user-space using absolute
timeouts and sending UDP packets.
See audio/hpsjam for more details. The test program was run using real-time
priority and the resulting jitter was measured by ear.
MFC after: 1 week
Sponsored by: NVIDIA Networking