[reboot] add new hw reason for user-initiated hard reset

* this corresponds to when the user can press a key or insert a pin
  that the hardware is set up to recognize as a need for a hard reboot
  without involving Fuchsia
* we don't actually set it in any codepaths yet

Bug: 494585004
Change-Id: I79433f8188857de7cedb2fae928d9a4606e1921b
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1550821
Commit-Queue: Francois Rousseau <frousseau@google.com>
Reviewed-by: David Pursell <dpursell@google.com>
API-Review: Roland McGrath <mcgrathr@google.com>
Reviewed-by: John Grossman <johngro@google.com>
Reviewed-by: Roland McGrath <mcgrathr@google.com>