Add poison cloud glyph, fumaroles to fire plane.

When a gas cloud that deals damage is created, it uses
a poison cloud glyph instead of the cloud glyph.
(A bright green '#', or a bright-green recolor of the
cloud tile)

The plane of fire has random "stinking clouds", or
fumaroles, centered on lava pools.

Also make poison cloud glyph override lava, pool and
moat glyphs.

src/mkmaze.c

@@ -879,6 +879,26 @@ register xchar x, y, todnum, todlevel;
 	return;
 }
 
+void
+fumaroles()
+{
+    xchar n;
+    boolean snd = FALSE, loud = FALSE;
+    for (n = rn2(3)+2; n; n--) {
+	xchar x = rn1(COLNO-4,3);
+	xchar y = rn1(ROWNO-4,3);
+	struct trap *ttmp = t_at(x,y);
+	if (levl[x][y].typ == LAVAPOOL) {
+	    NhRegion *r = create_gas_cloud(x,y, 4+rn2(5), rn1(10,5));
+	    clear_heros_fault(r);
+	    snd = TRUE;
+	    if (distu(x,y) < 15) loud = TRUE;
+	}
+    }
+    if (snd && !Deaf)
+	Norep("You hear a %swhoosh!", loud ? "loud " : "");
+}
+
 /*
  * Special waterlevel stuff in endgame (TH).
  *