vampires now shapeshift [trunk only]

- can shift into fog clouds, vampire bats, and vampire lords into wolves
- after being "killed" in shifted form, they transform back rather than get
  destroyed, and you must take them on in vampire form to defeat them
- can deliberately shift into fog clouds to pass under closed doors

src/potion.c

@@ -1211,7 +1211,7 @@ boolean your_fault;
 		break;
 	case POT_WATER:
 		if (is_undead(mon->data) || is_demon(mon->data) ||
-			is_were(mon->data)) {
+			is_were(mon->data) || is_vampshifter(mon)) {
 		    if (obj->blessed) {
 			pline("%s %s in pain!", Monnam(mon),
 			      is_silent(mon->data) ? "writhes" : "shrieks");