whence -f: ignore functions (#137)

According to 'whence --man', 'whence -f' should ignore functions:
  -f              Do not check for functions.

Right now this is only accomplished partially. As of commit
a329c22d 'whence -f' avoids any output when encountering a
function (in ksh93u+ 'whence -f' has incorrect output). The
return value is still wrong though:

$ foo() { true; }
$ whence -f foo; echo $?
0

This commit fixes the return value and makes 'type -f' error out
when given a function (like in Bash).

src/cmd/ksh93/bltins/whence.c:
- If -f was passed, set 'cp' to NULL since functions should be
  ignored (as documented).
- Simplify return value by avoiding bitwise logic.

src/cmd/ksh93/tests/builtins.sh:
- Add regression tests for 'whence -f' and 'type -f'.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

NEWS

@@ -3,6 +3,10 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-26:
+
+- 'whence -f' now completely ignores the existence of functions, as documented.
+
 2020-09-25:
 
 - whence -v/-a now reports the path to the file that an "undefined" (i.e.