mksh/ksh MirBSD ksh version available from shell variable $KSH_VERSION

[BrianInglis]

The mksh/ksh MirBSD portable ksh version is available from the shell variable $KSH_VERSION.
This should be added to the lib/versions script in a suitably portable manner, and also support for /bin/mksh?
As this is on a Cygwin system, it is currently finding the mksh MS Windows manifest version instead, which is amusing, but not helpful.
This variable was found using strings /bin/ksh | grep -i version and confirmed from the command and man page.

$ /bin/ksh -c 'echo $KSH_VERSION'
@(#)MIRBSD KSH R56 2018/01/14
$ /bin/mksh -c 'echo $KSH_VERSION'
@(#)MIRBSD KSH R56 2018/01/14
$ man mksh
...
KSH_VERSION The name and version of the shell (read‐only).
See also the version  commands  in  “Emacs  editing mode” and
“Vi editing mode” sections, below.
...
Emacs editing mode
...
version: ^[^V
Display the version of mksh.
The current edit buffer is restored as soon as  a  key  is  pressed.
The restoring keypress is processed, unless it is a space.
...
Vi editing mode
...
^V Display the version of mksh.
The current edit buffer is restored as soon as a key is pressed.
The restoring keypress is ignored.

You might also want to consider supporting posh:

$ /bin/posh -c 'echo $POSH_VERSION'
0.14.1
$ man posh
...
NAME
	posh - Policy-compliant Ordinary SHell
...
DESCRIPTION
	posh is a reimplementation of the Bourne shell,
	a command interpreter for both interactive and script use.
...
AUTHORS
	This shell is based on pdksh.