found_artifact() groundwork

Lay groundwork for generating a log event when finding an artifact
on the floor or carried by a monster.  This part should not produce
any change in behavior.

Move g.artidisco[] and g.artiexist[] out of the instance_globals
struct back to local within artifact.c.  They are both initialized
at the start of a game (and only used in that file) so don't need
to be part of any bulk reinitialization if restart-instead-of-exit
ever gets implemented.

Convert artiexist[] from an array of booleans to an array of structs
containing a pair of bitfields.  artiexist[].exists is a direct
replacement for the boolean; artiexist[].found is new but not put to
any significant use yet.  If will be used to suppress the future
found-an-artifact event for cases where a more specific event (like
crowning or divine gift as #offer reward) is already produced.

Remove g.via_naming altogether and add an extra argument to oname()
calls to replace it.

Add an extra argument to artifact_exists() calls.

src/pickup.c

@@ -2609,7 +2609,7 @@ observe_quantum_cat(struct obj *box, boolean makecat, boolean givemsg)
                now rather than from when this special corpse got created */
             deadcat->age = g.moves;
             set_corpsenm(deadcat, PM_HOUSECAT);
-            deadcat = oname(deadcat, sc);
+            deadcat = oname(deadcat, sc, ONAME_NO_FLAGS);
         }
         if (givemsg)
             pline_The("%s inside the box is dead!",