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)

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.