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/decl.c

@@ -202,8 +202,6 @@ const struct instance_globals g_init = {
 
     /* artifact.c */
     0,  /* spec_dbon_applies */
-    UNDEFINED_VALUES, /* artiexist */
-    UNDEFINED_VALUES, /* artdisco */
     0, /* mkot_trap_warn_count */
 
     /* botl.c */
@@ -352,7 +350,6 @@ const struct instance_globals g_init = {
     /* do_name.c */
     NULL, /* gloc_filter_map */
     UNDEFINED_VALUE, /* gloc_filter_floodfill_match_glyph */
-    0, /* via_naming */
 
     /* do_wear.c */
     FALSE, /* initial_don */