Scale poison needles with throwing skill

Instead of having poison needles do a fixed amount of poison, now the
amount of poison delivered scales heavily with Throwing skill and slightly
less heavily with launcher enchantment.

The practical impact of this is that poison needles are worse at very low
skill, break even at around 8 skill, and then are better at higher skill.
And also blowgun enchantment matters.

crawl-ref/source/ranged_attack.cc

@@ -580,7 +580,8 @@ bool ranged_attack::blowgun_check(special_missile_type type)
 
 int ranged_attack::blowgun_duration_roll(special_missile_type type)
 {
-    if (type == SPMSL_POISONED)
+    // Leaving monster poison the same by separating it from player poison
+    if (type == SPMSL_POISONED && attacker->is_monster())
         return 6 + random2(8);
 
     if (type == SPMSL_CURARE)
@@ -610,6 +611,8 @@ int ranged_attack::blowgun_duration_roll(special_missile_type type)
                 return 5 + random2(5);
         }
     }
+    else if (type == SPMSL_POISONED) // Player poison needles
+        return random2(3 + base_power * 2 + plus);
     else
         return 5 + random2(base_power + plus);
 }