tweak: make better use of funptr() macro and sh.bltinfun pointer

src/cmd/ksh93/include/name.h:
- Include the ususally-wanted (Shbltin_f) typecast in funptr().

Various files:
- Change several direct foo->nvalue.bfp usages to funptr(np).
- Reduce explicit typecasts after the name.h change.
- To determine if we are (or just were) running a certain built-in
  command, instead of comparing sh.bltindata.bnode with a builtin
  table node, use sh.bltinfun to directly compare the builtin's
  function; this is more readable and efficient.

src/cmd/ksh93/include/name.h

@@ -149,7 +149,7 @@ struct Ufunction
 #define is_abuiltin(n)	(nv_isattr(n,NV_BLTIN|NV_INTEGER)==NV_BLTIN)
 #define is_afunction(n)	(nv_isattr(n,NV_FUNCTION|NV_REF)==NV_FUNCTION)
 #define	nv_funtree(n)	((n)->nvalue.rp->ptree)
-#define	funptr(n)	((n)->nvalue.bfp)
+#define	funptr(n)	((Shbltin_f)(n)->nvalue.bfp)
 
 /* NAMNOD MACROS */
 /* ... for attributes */