Reduce minimal time intervals of setitimer(2) from 1/HZ to 1/(16*HZ) by
using callout_reset_sbt() instead of callout_reset().  We can't remove
lower limit completely in this case because of significant processing
overhead, caused by unability to use direct callout execution due to using
process mutex in callout handler for sending SEGALRM signal.  With support
of periodic events that would allow unprivileged user to abuse the system.

Reviewed by:	davide
Remove strnvis(), strvis(), strvisx().