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)

NEWS

@@ -17,6 +17,12 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Completed the 2021-04-30 fix for ${var<OP>'{}'} where <OP> is '-', '+',
   ':-' or ':+' by fixing a bug that caused an extra '}' to be output.
 
+- 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. This reverts a change introduced on 2020-09-11.
+  [*] https://austingroupbugs.net/view.php?id=1393
+
 2021-04-30:
 
 - The emacs 'ESC .' (M-.) and vi '_' commands now take shell quoting into