restore C90 compat: do not repeat a typedef (re: 3e0da770)

We try to stay compatibile with C90. Turns out that repeating a
typedef is valid only from C11 onwards, as a feature taken from
C++. So I goofed and broke the build on old or strict compilers.

src/cmd/ksh93/include/{name,shell}.h:
- union Value: Since we will now once again have to typecast to use
  nvalue.bfp in any case, just make it a void pointer; that is how
  pointers that require typecasts are handled in every other case.
- Since the funptr() macro needs a typecast to Shbltin_f which is
  defined in libast's shcmd.h, move this macro to shell.h which
  (unlike name.h) includes that header.

src/cmd/ksh93/sh/{init,nvdisc}.c:
- Typecast to void* when assigning to *->nvalue.bfp.

src/lib/libast/include/shcmd.h:
- Use shell_h_defined (introduced in 4491bc6a) and defs_h_defined
  to check if ksh's shell.h or defs.h were included before shcmd.h,
  instead of random macros defined by them; much clearer.

src/cmd/ksh93/include/name.h

@@ -31,9 +31,6 @@
 #include	<ast.h>
 #include	<cdt.h>
 
-typedef struct Shbltin_s Shbltin_t;
-typedef int (*Shbltin_f)(int, char**, Shbltin_t*);
-
 /* Nodes can have all kinds of values */
 union Value
 {
@@ -55,7 +52,7 @@ union Value
 	struct Ufunction 	*rp;	/* shell user defined functions */
 	struct Namfun		*funp;	/* discipline pointer */
 	struct Namref		*nrp;	/* name reference */
-	Shbltin_f		bfp;	/* builtin entry point function pointer */
+	void			*bfp;	/* pointer to built-in command's entry function (typecast to Shbltin_f) */
 };
 
 #include	"nval.h"
@@ -150,7 +147,6 @@ 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)
 
 /* NAMNOD MACROS */
 /* ... for attributes */

src/cmd/ksh93/include/shell.h

@@ -52,6 +52,9 @@ typedef struct Shell_s Shell_t;
 
 #include	<shcmd.h>
 
+/* get pointer to a built-in command's entry function */
+#define	funptr(n)	((Shbltin_f)(n)->nvalue.bfp)
+
 typedef void	(*Shinit_f)(Shell_t*, int);
 #ifndef SH_wait_f_defined
     typedef int	(*Shwait_f)(int, long, int);