Exit the process on game-over instead of unwinding a panic

One game run is one process, so game-over ends the process directly,
as the C game did with exit(). myExit now restores the terminal
(via the new Terminal.Fini) and calls os.Exit(0); the gameEnd sentinel,
the recover in Run, and the recover in DeathDemo are gone. Run() no
longer returns an error (it does not return — the game exits from
within), and playit's pre-loop setup is split into startLevel/prePlay
so tests can drive a bounded number of turns.

Because death (combat, and starvation over a long session) now exits
the process, the four Run()-to-completion tests can no longer run
through the exit path: TestDeathUnwindsWithGameEnd is removed (it
tested the deleted unwind), the crash-sweep and quit/save session
tests are dropped, and TestRunDownStairs is reworked to drive the
turn loop for a single descend. Score rendering, previously checked
after a scripted quit, is now covered directly by TestScoreRendersList.
Save/restore integrity remains covered by TestSaveRestoreRoundTrip.

game/fight_test.go

@@ -79,24 +79,6 @@ func TestAttackHurtsPlayer(t *testing.T) {
 	}
 }
 
-func TestDeathUnwindsWithGameEnd(t *testing.T) {
-	g := mkGame(t, 11)
-
-	defer func() {
-		r := recover()
-		if _, ok := r.(gameEnd); !ok {
-			t.Fatalf("death did not unwind with gameEnd, got %v", r)
-		}
-
-		if g.Playing {
-			t.Error("still playing after death")
-		}
-	}()
-
-	g.Options.Tombstone = false
-	g.death('K')
-}
-
 func TestRunnersChaseHero(t *testing.T) {
 	g := mkGame(t, 3)
 	// Place a hobgoblin a few squares away in the hero's room and set it