Random item artifact chances

The chance that randomly generated item is an artifact will
now depend on the number of already existing artifacts.

doc/fixes3-7-0.txt

@@ -992,6 +992,7 @@ make healing and extra healing better by upping the average amount healed
 lifesaving healing amount depends on the consitution
 hitting a monster with a wielded potion hits you far less
 pyrolisks get a mundane bite attack
+chances of random item being an artifact depends on already existing artifacts
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

src/mkobj.c

@@ -817,7 +817,7 @@ mksobj(int otyp, boolean init, boolean artif)
             if (is_poisonable(otmp) && !rn2(100))
                 otmp->opoisoned = 1;
 
-            if (artif && !rn2(20))
+            if (artif && !rn2(20 + (10 * nartifact_exist())))
                 otmp = mk_artifact(otmp, (aligntyp) A_NONE);
             break;
         case FOOD_CLASS:
@@ -1020,7 +1020,7 @@ mksobj(int otyp, boolean init, boolean artif)
                 otmp->spe = rne(3);
             } else
                 blessorcurse(otmp, 10);
-            if (artif && !rn2(40))
+            if (artif && !rn2(40 + (10 * nartifact_exist())))
                 otmp = mk_artifact(otmp, (aligntyp) A_NONE);
             /* simulate lacquered armor for samurai */
             if (Role_if(PM_SAMURAI) && otmp->otyp == SPLINT_MAIL