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/lib/libast/include/shcmd.h

@@ -69,7 +69,7 @@ struct Shbltin_s
 	int		invariant;
 };
 
-#if defined(SH_VERSION) ||  defined(_SH_PRIVATE)
+#if defined(shell_h_defined) || defined(defs_h_defined)
 #   undef Shell_t
 #   undef Namval_t
 #else