[[ ... ]]: fix '!' to negate another '!'

Bug: [[ ! ! 1 -eq 1 ]] returns false, but should return true.

This bug was reported for bash, but ksh has it too:
https://lists.gnu.org/archive/html/bug-bash/2021-06/msg00006.html

Op 24-05-21 om 17:47 schreef Chet Ramey:
> On 5/22/21 2:45 PM, Vincent Menegaux wrote:
>> Previously, these commands:
>>
>>    [[ ! 1 -eq 1 ]]; echo $?
>>    [[ ! ! 1 -eq 1 ]]; echo $?
>>
>> would both result in `1', since parsing `!' set CMD_INVERT_RETURN
>> instead of toggling it.
>
> Interestingly, ksh93 produces the same result as bash. I agree
> that it's more intuitive to toggle it.

Also interesting is that '!' as an argument to the simple
'test'/'[' command does work as expected (on both bash and ksh93):
'test ! ! 1 -eq 1' and '[ ! ! 1 -eq 1 ]' return 0/true.

Even the man page for [[ is identical for bash and ksh93:

|               ! expression
|                      True if expression is false.

This suggests it's supposed to be a logical negation operator, i.e.
'!' is implicitly documented to negate another '!'. Bolsky & Korn's
1995 ksh book, p. 167, is slightly more explicit about it:
"! test-expression. Logical negation of test-expression."

I also note that multiple '!' negators in '[[' work as expected on
mksh, yash and zsh.

src/cmd/ksh93/sh/parse.c: test_primary():
- Fix bitwise logic for '!': xor the TNEGATE bit into tretyp
  instead of or'ing it, which has the effect of toggling it.

src/cmd/ksh93/tests/bracket.sh

@@ -411,5 +411,17 @@ unset foo
 foo=10
 ([[ foo -eq 10 ]]) || err_exit 'foo -eq 10 fails in [[ ... ]] with foo=10'
 
+# ======
+# The negator should negate the negator
+# This bug was shared with bash:
+# https://lists.gnu.org/archive/html/bug-bash/2021-06/msg00006.html
+[[ ! ! -n x ]] && ! [[ ! ! ! -n x ]] && [[ ! ! ! ! -n x ]] && ! [[ ! ! ! ! ! -n x ]] \
+&& [[ ! ! -n x && ! ! ! ! -n x && ! ! ! ! ! ! -n x ]] \
+|| err_exit '! does not negate ! in [[ ... ]]'
+# The bug did not exist in 'test'/'[', but check for it anyway
+[ ! ! -n x ] && ! [ ! ! ! -n x ] && [ ! ! ! ! -n x ] && ! [ ! ! ! ! ! -n x ] \
+&& [ ! ! -n x -a ! ! ! ! -n x -a ! ! ! ! ! ! -n x ] \
+|| err_exit '! does not negate ! in [ ... ]'
+
 # ======
 exit $((Errors<125?Errors:125))