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.

include/decl.h

@@ -421,7 +421,6 @@ struct plinemsg_type {
 /* bitmask for callers of hide_unhide_msgtypes() */
 #define MSGTYP_MASK_REP_SHOW ((1 << MSGTYP_NOREP) | (1 << MSGTYP_NOSHOW))
 
-
 enum bcargs {override_restriction = -1};
 struct breadcrumbs {
     const char *funcnm;
@@ -701,10 +700,6 @@ struct instance_globals {
     /* artifcat.c */
     int spec_dbon_applies; /* coordinate effects from spec_dbon() with
                               messages in artifact_hit() */
-    /* flags including which artifacts have already been created */
-    boolean artiexist[1 + NROFARTIFACTS + 1];
-    /* and a discovery list for them (no dummy first entry here) */
-    xchar artidisco[NROFARTIFACTS];
     int mkot_trap_warn_count;
 
     /* botl.c */
@@ -864,7 +859,6 @@ struct instance_globals {
     /* do_name.c */
     struct selectionvar *gloc_filter_map;
     int gloc_filter_floodfill_match_glyph;
-    int via_naming;
 
     /* do_wear.c */
     /* starting equipment gets auto-worn at beginning of new game,