POSIX: 'command': don't disable declaration proprts (re: b9d10c5a)

Following the resolution of Austin Group bug 1393[*] that is set to
be included in the next version of the POSIX standard, the
'command' prefix in POSIX mode (set -o posix) no longer disables
the declaration properties of declaration built-ins.
[*] https://austingroupbugs.net/view.php?id=1393

src/cmd/ksh93/sh/parse.c: lex():
- Skip the 'command' prefix even in POSIX mode so that any
  declaration commands prefixed by it are treated as such in xec.c
  (sh_exec()).

src/cmd/ksh93/sh/xec.c: sh_exec():
- The foregoing change reintroduced a variant of BUG_CMDSPEXIT: the
  shell exits on something like 'command export readonlyvar=foo'.
  This now fixes that bug for both POSIX and non-POSIX mode. When
  calling nv_setlist() to process true shell assignments, and there
  is a 'command' prefix, push a shell context and use sigsetjmp to
  intercept any errors in assignments and stop the shell exiting.

src/cmd/ksh93/tests/builtins.sh:
- Borrow the BUG_CMDSPEXIT regression test from modernish and adapt
  it for ksh. (I'm the author so yes, I can do this.) Original:
  ae8fe9c3/lib/modernish/tst/builtin.t (L80-L109)

src/cmd/ksh93/tests/builtins.sh

@@ -1241,5 +1241,30 @@ got=$("$SHELL" -c 'cd /; echo "$OLDPWD"' 2>&1)
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 cd "$tmp"
 
+# ======
+# BUG_CMDSPEXIT
+exp='ok1ok2ok3ok4ok5ok6ok7ok8ok9ok10ok11ok12end'
+got=$(	readonly v=foo
+	exec 2>/dev/null
+	# All the "special builtins" below should fail, and not exit, so 'print end' is reached.
+	# Ref.: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html
+	# Left out are 'command exec /dev/null/nonexistent', where no shell follows the standard,
+	# as well as 'command exit' and 'command return', because, well, obviously.
+	command : </dev/null/no		|| print -n ok1
+	command . /dev/null/no		|| print -n ok2
+	command set +o bad@option	|| print -n ok3
+	command shift $(($# + 1))	|| print -n ok4
+	(unalias times; PATH=/dev/null; eval 'command times foo bar >/dev/null || print -n ok5')
+	command trap foo bar baz quux	|| print -n ok6
+	command unset v			|| print -n ok7
+	command eval "("		|| print -n ok8
+	command export v=baz		|| print -n ok9
+	command readonly v=bar		|| print -n ok10
+	command break			&& print -n ok11  # 'break' and 'continue' are POSIXly allowed to quietly...
+	command continue		&& print -n ok12  # ..."succeed" if they are used outside of a loop :-/
+	print end)
+[[ $got == "$exp" ]] || err_exit "prefixing special builtin with 'command' does not stop it from exiting the shell on error" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))