kern/subr_trap.c: repair the HPTS performance hack in userret()

[glebius]

Summary

It wasn't functional as subr_trap.c doesn't include opt_inet.h. Put a
better comment in place of the old one. The idea is to use userret() as
a cheap place to call a soft clock. This approach saves CPU on busy
machines and saves power on idle machines. An alternative would be to
constantly schedule callouts. Running with neither callouts nor the soft
clock ruins HPTS precision.