Add more explicit helpless reasons

Instead of just "while helpless", the death reason will tell
more explicitly why the player was helpless.  For example:
"while frozen by a monster's gaze"

src/mcastu.c

@@ -634,12 +634,14 @@ int spellnum;
 	    if (multi >= 0)
 		You("stiffen briefly.");
 	    nomul(-1);
+	    multi_reason = "paralyzed by a monster";
 	} else {
 	    if (multi >= 0)
 		You("are frozen in place!");
 	    dmg = 4 + (int)mtmp->m_lev;
 	    if (Half_spell_damage) dmg = (dmg + 1) / 2;
 	    nomul(-dmg);
+	    multi_reason = "paralyzed by a monster";
 	}
 	nomovemsg = 0;
 	dmg = 0;