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