Decrease SHLVL before doing 'exec' from main shell

Problem:

$ exec ksh
$ echo $SHLVL
2
$ exec ksh
$ echo $SHLVL
3
$ exec ksh
$ echo $SHLVL
4

...etc. SHLVL is supposed to acount the number of shell processes
that you need to exit before you get logged out. Since ksh was
replacing itself with a new shell in the same process using 'exec',
SHLVL should not increase.

src/cmd/ksh93/bltins/misc.c: b_exec():
- When about to replace the shell and we're not in a subshell,
  decrease SHLVL to cancel out a subsequent increase by the
  replacing shell. Bash and zsh also do this.

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.
 
+2021-05-18:
+
+- Fixed SHLVL so that replacing ksh by itself (exec ksh) will not increase it.
+
 2021-05-13:
 
 - Fixed a bug with 'test -t 1' that was introduced on 2021-04-26: