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:

src/cmd/ksh93/bltins/misc.c

@@ -161,6 +161,9 @@ int    b_exec(int argc,char *argv[], Shbltin_t *context)
 		if(job_close(logdata.sh) < 0)
 			return(1);
 #endif /* JOBS */
+		/* if the main shell is about to be replaced, decrease SHLVL to cancel out a subsequent increase */
+		if(!shgd->realsubshell)
+			(*SHLVL->nvalue.ip)--;
 		/* force bad exec to terminate shell */
 		pp = (struct checkpt*)logdata.sh->jmplist;
 		pp->mode = SH_JMPEXIT;

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-05-13"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-05-18"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh.1

@@ -1802,8 +1802,8 @@ be the value that was assigned plus the number of seconds since the assignment.
 .TP
 .SM
 .B SHLVL
-An integer variable the is incremented each time the shell
-is invoked and is exported.
+An integer variable that is incremented and exported
+each time the shell is invoked.
 If
 .SM
 .B SHLVL
@@ -6028,6 +6028,11 @@ is given,
 the command specified by
 the arguments is executed in place of this shell
 without creating a new process.
+The value of the
+.SM
+.B SHLVL
+environment variable is decreased by one,
+unless the shell replaced is a subshell.
 The
 .B \-c
 option causes the environment to be cleared before applying