Allow stack(9) to capture stack of running process via
stack_save_thread().  Have kern.proc.kstack use this.

If a process is running sleep until hardclock can capture its stack.

TODO: non-amd64 support, stack.9 updates, API fixes as now there is both
      stack_save_td() and stack_save_thread()

Sponsored by:	EMC / Isilon Storage Division
Obtained from:	Isilon OneFS
Phabric:	D323