POSIX compliance fix: apply 'set -u' to $!

POSIX requires[*] that expanding any unset parameter other than $@
and $* is an error when 'set -u'/'set -o nounset' is active.
However, on ksh93, $! was 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:
- special(): Handle 'set -u' for special parameters if/when it is
  about to return NIL. That code path is currently only possible to
  reach for "$!", but this is future-proof and will do the right
  thing if any other special parameter can ever have no value.

src/cmd/ksh93/tests/options.sh:
- Add and tweak 'set -u' regression tests.

(cherry picked from commit 75cc7a38cafe3a9929e1ed17d8b952babda22a09)

NEWS

@@ -10,6 +10,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   nonexistent positional parameter such as $1, $2, ... is accessed, as other
   shells do and POSIX requires. (This does *not* apply to "$@" and "$*".)
 
+- If 'set -u'/'set -o nounset' is active, then the shell now errors out if $!
+  is accessed before the shell has launched any background process.
+
 2020-06-06:
 
 - The 'times' command is now a builtin command that conforms to POSIX