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/insight.c

@@ -2311,6 +2311,9 @@ record_achievement(schar achidx)
                       ? g.context.achieveo.soko_prize_otyp
                       : g.context.achieveo.mines_prize_otyp);
 
+        /* note: OBJ_NAME() works here because both "bag of holding" and
+           "amulet of reflection" are fully named in their objects[] entry
+           but that's not true in the general case */
         livelog_printf(achieve_msg[achidx].llflag, "%s %s",
                        achieve_msg[achidx].msg, OBJ_NAME(objects[otyp]));
     } else {