combine taming of already tame monsters

Merge the recent change in the effect of blessed scroll of taming on
already tame monsters with the earlier change of any taming on already
tame monsters.  Non-blessed has a chance of boosting monst->mtame by 1
when it is less than 10, more likely the lower the current value is.
For blessed, boost by 2 after that, so possibly by 3 if it is very low.

Make spell of charm monster when skilled or expert in enchantment
spells behave the same as blessed scroll of taming.  [I'm not too sure
about this; it may make the spell too powerful.]

src/spell.c

@@ -1509,12 +1509,12 @@ spelleffects(int spell_otyp, boolean atme, boolean force)
     case SPE_DETECT_FOOD:
     case SPE_CAUSE_FEAR:
     case SPE_IDENTIFY:
+    case SPE_CHARM_MONSTER:
         /* high skill yields effect equivalent to blessed scroll */
         if (role_skill >= P_SKILLED)
             pseudo->blessed = 1;
         FALLTHROUGH;
     /*FALLTHRU*/
-    case SPE_CHARM_MONSTER:
     case SPE_MAGIC_MAPPING:
     case SPE_CREATE_MONSTER:
         (void) seffects(pseudo);