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/makemon.c

@@ -326,7 +326,8 @@ m_initweap(register struct monst *mtmp)
             /* maybe make it special */
             if (!rn2(20) || is_lord(ptr))
                 otmp = oname(otmp,
-                             artiname(rn2(2) ? ART_DEMONBANE : ART_SUNSWORD));
+                             artiname(rn2(2) ? ART_DEMONBANE : ART_SUNSWORD),
+                             ONAME_NO_FLAGS);
             bless(otmp);
             otmp->oerodeproof = TRUE;
             otmp->spe = rn2(4);