whence -v/-a: report path to autoloadable functions

Since at least 1999, whence -v on pdksh (and its successor mksh)
reports the path where an autoloadable function may be found:

  $ mkdir ~/fun; FPATH=~/fun
  $ echo 'myfn() { echo hi; }' >~/fun/myfn
  $ whence -v myfn
  myfn is a undefined (autoload from /home/user/fun/myfn) function

Whereas ksh93 only reports, rather uselessly:

  myfn is an undefined function

As of this commit, whence -v/-a on ksh 93u+m does the same as
pdksh, but with correct grammar:

  myfn is an undefined function (autoload from /home/user/fun/myfn)

This may be a small violation of my own "no new features" policy
for 93u+m, but I couldn't resist. This omission has been annoying
me, and it's just embarrassing to lack a pdksh feature :)

src/cmd/ksh93/include/path.h,
src/cmd/ksh93/data/msg.c:
- Add e_autoloadfrom[] = " (autoload from %s)" message.

src/cmd/ksh93/bltins/whence.c: whence():
- Report the path (if any) when reporting an undefined function.
  This needs to be done in two places:
  1. When a function has been explicitly marked undefined with
     'autoload', we need to do a quick path_search() loop to find
     the path. (These undefined functions take precedence over
     regular commands, so are reported first.)
  2. When a function is not explicitly autoloaded but merely
     available in $FPATH, that path search was already done, so all
     we need to do is report it. (These are reported last.)
  Note that the output remains as on 93u+ if no function definition
  file is found on $FPATH. This is also like pdksh/mksh.

src/cmd/ksh93/data/builtins.c:
- Bump 'whence' version date. The inline docs never detailed very
  exactly what 'whence -v' reports, so no need for further edits.

src/cmd/ksh93/tests/path.sh:
- Regress-test the new whence behaviour plus actual autoloading,
  including the command override behaviour of autoloaded functions.

src/cmd/ksh93/data/builtins.c

@@ -1988,7 +1988,7 @@ _JOB_
 ;
 
 const char sh_optwhence[] =
-"[-1c?\n@(#)$Id: whence (AT&T Research/ksh93) 2020-07-20 $\n]"
+"[-1c?\n@(#)$Id: whence (AT&T Research/ksh93) 2020-09-25 $\n]"
 USAGE_LICENSE
 "[+NAME?whence, type - locate a command and describe its type]"
 "[+DESCRIPTION?Without \b-v\b, \bwhence\b writes on standard output an "