KASSERT we don't kpause indefinitely without interruptability.

XXX: using timo == 0 to mean "sleep as long as you like, and forever
if you're really tired" is not the smartest interface considering
the the hz/n idiom used to specify timo.  This leads to unwanted
behaviour when hz gets below some impossible-to-know limit.  With
a usec2ticks() routine it at least be a little more tolerable.

(pooka)