whence -a/type -a: report both function and built-in by same name

'whence -a' is documented to list all possible interpretations of a
command, but failed to list a built-in command if a shell function
by the same name exists or is marked undefined using 'autoload'.

src/cmd/ksh93/bltins/whence.c: whence():
- Refactor and separate the code for reporting functions and
  built-in commands so that both can be reported for one name.

src/cmd/ksh93/data/builtins.c: sh_optwhence[]:
- Correct 'whence --man' to document that:
  * 'type' is equivalent to 'whence -v'
  * '-a' output is like '-v'

src/cmd/ksh93/tests/builtins.sh:
- Test 'whence -a' with these combinations:
  * a function, built-in and external command
  * an undefined/autoload function, built-in and external command

Fixes https://github.com/ksh93/ksh/issues/83

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-07-19"
+#define SH_RELEASE	"93u+m 2020-07-20"