Power attack, and some notification-related cleanup.

Power attack, and some notification-related cleanup.

* combat.cc,combat.hh: Implement power attack and refactor uhitm().
* map.cc: Replace touch_back_buffer() with notify_fov()
* notify-local-tty.cc, notify.hh: Various new notifications.
* objects.cc: Reflow eat_food(), eliminate reference to display vars therein.
* u.cc: Debug chain-of-detection for power attack