redirect: check args before executing redirections (re: 7b82c338)

The 'redirect' builtin command did not error out before executing
any valid redirections. For example, 'redirect ls >foo.txt' issued
an "incorrect syntax" error, but still created 'foo.txt' and left
standard output permanently redirected to it.

src/cmd/ksh93/sh/xec.c: sh_exec():
- If we have redirections (io != NULL), and the command is
  SYSREDIR, then check for arguments and error out if there are
  any, before calling sh_redirect() to execute redirections.
  (Note, the other check for arguments in b_exec() in bltins/misc.c
  must be kept, as that applies if there are no redirections.)

src/cmd/ksh93/sh/io.c: sh_redirect():
- Edit comments to better explain what the flag values do.

src/cmd/ksh93/bltins/misc.c:
- Add a dummy b_redirect() function declaration "for the dictionary
  generator" as has historically been done for other builtins that
  share one C function. I'm not sure what that dictionary generator
  is supposed to be, but this also improves greppability.

src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/sh.1:
- Fix misleading "I/O redirection arguments" term. I/O redirections
  are not arguments at all; no argument parser ever sees them.

src/cmd/ksh93/tests/io.sh:
- Test both conditions that should make 'redirect' produce an
  "incorrect syntax" error.
- Test that any redirections are not executed if erroneous
  non-redirection arguments exist.

src/cmd/ksh93/tests/builtins.sh:
- "... should show usage info on unrecognized options" test:
  Because 'redirect' now refuses to process redirections on error,
  the error message was not captured. The fix is to run the builtin
  in a braces block and add the redirection to the block.

NEWS

@@ -3,6 +3,13 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-08-08:
+
+- Argument checking in the 'redirect' builtin command (see 2020-06-11) has
+  been improved to error out before executing redirections. For example, an
+  error like 'redirect ls >foo.txt' now will not create 'foo.txt' and will
+  not leave your standard output permanently redirected to it.
+
 2020-08-07:
 
 - Fixed a crash that occurred intermittently when entering the 60 second