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/sh/parse.c

@@ -1456,7 +1456,7 @@ static Shnode_t *simple(Lex_t *lexp,int flag, struct ionod *io)
 							lexp->intypeset = 1;
 						key_on = 1;
 					}
-					else if(np==SYSCOMMAND && !sh_isoption(SH_POSIX))
+					else if(np==SYSCOMMAND)	/* treat 'command typeset', etc. as declaration command */
 						cmdarg++;
 					else if(np==SYSEXEC || np==SYSREDIR)
 						lexp->inexec = 1;