Fire/Frost Brand vs rusting

Implement the suggestion that Fire Brand avoids damage from rust traps
by boiling away the water.  Rather than making this be trap-specific, it
applies to all types of erosion which go through erode_obj().  That includes
hitting rust monsters but not dipping into potions or fountains, nor falling
into moats.  And it doesn't provide 100% protection, just a high chance of
avoiding rust damage.  Also, Frost Brand gets similar protection by freezing.

     The message handling needed some rewriting for the branch version.
That compiles ok but hasn't been tested.  It would have been simpler just to
move Yobjnam2() over even if nothing else was changed to use it yet....

include/extern.h

@@ -60,6 +60,7 @@ E const char *FDECL(artifact_name, (const char *,short *));
 E boolean FDECL(exist_artifact, (int,const char *));
 E void FDECL(artifact_exists, (struct obj *,const char *,BOOLEAN_P));
 E int NDECL(nartifact_exist);
+E boolean FDECL(arti_immune, (struct obj *,int));
 E boolean FDECL(spec_ability, (struct obj *,unsigned long));
 E boolean FDECL(confers_luck, (struct obj *));
 E boolean FDECL(arti_reflects, (struct obj *));