Conflict based off of charisma

Higher charisma will make it more likely for monsters to be affected.
Conflict will also now require the monster to see the hero.

Originally from SporkHack by Derek Ray.

src/mondata.c

@@ -1255,4 +1255,19 @@ monstseesu(unsigned long seenres)
             m_setseenres(mtmp, seenres);
 }
 
+/* Can monster resist conflict caused by hero?
+
+   High-CHA heroes will be able to 'convince' monsters
+   (through the magic of the ring, of course) to fight
+   for them much more easily than low-CHA ones.
+*/
+boolean
+resist_conflict(struct monst* mtmp)
+{
+    /* always a small chance at 19 */
+    int resist_chance = min(19, (ACURR(A_CHA) - mtmp->m_lev + u.ulevel));
+
+    return (rnd(20) > resist_chance);
+}
+
 /*mondata.c*/