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/mail.c

@@ -409,7 +409,7 @@ newmail(struct mail_info *info)
         struct obj *obj = mksobj(SCR_MAIL, FALSE, FALSE);
 
         if (info->object_nam)
-            obj = oname(obj, info->object_nam);
+            obj = oname(obj, info->object_nam, ONAME_NO_FLAGS);
         if (info->response_cmd)
             new_omailcmd(obj, info->response_cmd);