time: Allow higher-frequency POSIX timers

[tavianator_tavianator.com]

Summary

Commit b5ea3779da42 ("Reduce minimal time intervals of setitimer(2) from
1/HZ to 1/(16*HZ)") changed the minimal interval period for setitimer(2)
but not timer_settime(2). Update itimespecfix() to match itimerfix()
so that POSIX timers have equivalent functionality to the obsolete
setitimer() interface.

[imp]

(sorry for the lag... this got lost in the shuffle, alas)

This seems reasonable to me.

[imp]

I added mav@ since he can validate that this is a reasonable change since he did a lot of the tickless work.

[mav]

While the direction might be OK (I haven't looked close what is that timer), but I see realtimer_settime() calling the itimespecfix() still uses callout_reset(... tvtohz(&tv)), so it won't be able to handle intervals shorter than 1/HZ without bigger modification, similar to the mentioned b5ea3779da42.