fix "$-" expansion for posix option (re: 921bbcae)

In the SHOPT_BASH code, the -o posix option was given a '\374'
(0xFC, 252) single-letter option character. Reasons unclear. The
'set' builtin doesn't accept it. It can be omitted and the option
still works. And it caused the "$-" expansion (listing active
short-form options) to include that invalid high-bit character if
the -o posix option is active, which is clearly wrong.

src/cmd/ksh93/sh/args.c: optksh[], flagval[]:
- Remove '\374' one-letter option equivalent for SH_POSIX.

src/cmd/ksh93/tests/options.sh:
- Add test verifying that '-o posix' does not affect "$-".

src/cmd/ksh93/tests/options.sh

@@ -531,4 +531,15 @@ print '. ./dotfile' > envfile
 print $'alias print=:\nprint foobar' > dotfile
 [[ $(ENV=/.$PWD/envfile $SHELL -i -c : 2>/dev/null) == foobar ]] && err_exit 'files source from profile does not process aliases correctly'
 
+# ======
+# test that '-o posix' option (not having a letter) does not affect "$-" expansion
+(
+	command set +o posix 2>/dev/null
+	opt1=$-
+	command set -o posix 2>/dev/null
+	opt2=$-
+	[[ $opt1 == "$opt2" ]]
+) || err_exit '-o posix option affects $- expansion'
+
+# ======
 exit $((Errors<125?Errors:125))