POSIX compliance fix: apply 'set -u' to $1, $2, ...

POSIX requires[*] that expanding any unset parameter other than $@
and $* is an error when 'set -u'/'set -o nounset' is active.
However, on ksh93, $1, $2, ... were exempt as well. That is a bug.
[*] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_25

src/cmd/ksh93/sh/macro.c:
- varsub(): Backport code for handling 'set -u' for positional
  parameters from the ast 2016-10-01-beta branch.

src/cmd/ksh93/tests/options.sh:
- Add relevant regression tests.

src/cmd/ksh93/sh.1:
- Document that $@ and $* are exempt from 'set -u'.

(cherry picked from commit f954c6be0748c4c38a680a75f27564965fbd328e)

NEWS

@@ -4,6 +4,12 @@ For full details, see the git log at:
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-06-08:
+
+- If 'set -u'/'set -o nounset' is active, then the shell now errors out if a
+  nonexistent positional parameter such as $1, $2, ... is accessed, as other
+  shells do and POSIX requires. (This does *not* apply to "$@" and "$*".)
+
 2020-06-06:
 
 - The 'times' command is now a builtin command that conforms to POSIX

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-06-06"
+#define SH_RELEASE	"93u+m 2020-06-08"

src/cmd/ksh93/sh.1

@@ -7041,6 +7041,10 @@ Sort the positional parameters lexicographically.
 .TP 8
 .B \-u
 Treat unset parameters as an error when substituting.
+.B "$@"
+and
+.B "$\(**"
+are exempt.
 .TP 8
 .B \-v
 Print shell input lines as they are read.

src/cmd/ksh93/sh/macro.c

@@ -1195,6 +1195,14 @@ retry1:
 		}
 		else
 			v = 0;
+		/* Handle 'set -u'/'set -o nounset' for positional parameters */
+		if(!v && sh_isoption(SH_NOUNSET))
+		{
+			d=fcget();
+			fcseek(-1);
+			if(!(d && strchr(":+-?=",d)))
+				errormsg(SH_DICT,ERROR_exit(1),e_notset,ltos(c));
+		}
 		break;
 	    case S_ALP:
 		if(c=='.' && type==0)

src/cmd/ksh93/tests/options.sh

@@ -512,6 +512,9 @@ $SHELL -uc 'var=foo;unset var;: ${!var}' >/dev/null 2>&1 && err_exit '${!var} sh
 $SHELL -uc 'var=foo;unset var;: ${#var}' >/dev/null 2>&1 && err_exit '${#var} should fail with set -u'
 $SHELL -uc 'var=foo;unset var;: ${var-OK}' >/dev/null 2>&1 || err_exit '${var-OK} should not fail with set -u'
 $SHELL -uc 'var=foo;nset var;: ${var:-OK}' >/dev/null 2>&1 || err_exit '${var:-OK} should not fail with set -u'
+(set -u -- one two; : $2) 2>/dev/null || err_exit "an unset PP failed with set -u"
+(set -u -- one two; : $3) 2>/dev/null && err_exit "a set PP failed to fail with set -u"
+(set -u --; : $@ $*) 2>/dev/null || err_exit '$@ and/or $* fail to be exempt from set -u'
 
 z=$($SHELL 2>&1 -uc 'print ${X23456789012345}')
 [[ $z == *X23456789012345:* ]] || err_exit "error message garbled with set -u got $z"