- Change settime() to take a thread as its first argument instead of a proc

- Change settime() to take a thread as its first argument instead of a proc
  so it can use td_ucred.
- Push Giant down into the end of settime() where we actually set the time
  on the timecounter and time of day clock.
- Remove Giant from clock_settime().
- Push Giant down in settimeofday() to just protect the 'tz' global
  variable.