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.

src/cmd/ksh93/sh.1

@@ -5910,7 +5910,7 @@ to become
 for the new process.
 If
 .I arg\^
-is not given and only I/O redirection arguments are given,
+is not given and only I/O redirections are given,
 then this command persistently modifies file descriptors as in
 .BR redirect.
 .TP
@@ -6792,7 +6792,7 @@ When defining a type, if the value of a readonly sub-variable is not defined
 the value is required when creating each instance.
 .TP
 \f3redirect\fP
-This command only accepts input/output redirection arguments.
+This command only accepts input/output redirections.
 It can open and close files and modify file descriptors from
 .B 0
 to