- Add debug.watchdog tunable, so we can specify watchdog CPU from loader   which will help to debug hangs on boot. - Remove 'U' from debug.watchdog sysctl definition, so if we set it to '-1'   it really shows '-1'. - Fix comment.
- Add debug.watchdog tunable, so we can specify watchdog CPU from loader
  which will help to debug hangs on boot.
- Remove 'U' from debug.watchdog sysctl definition, so if we set it to '-1'
  it really shows '-1'.
- Fix comment.

Reviewed by:	rwatson

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