Make redirections like {varname}>file work with brace expansion off

This is some nonsense: redirections that store a file descriptor
greater than 9 in a variable, like {var}<&2 and the like, stopped
working if brace expansion was turned off. '{var}' is not a brace
expansion as it doesn't contain ',' or '..'; something like 'echo
{var}' is always output unexpanded. And redirections and brace
expansion are two completely unrelated things. It wasn't documented
that these redirections require the -B/braceexpand option, either.

src/cmd/ksh93/sh/lex.c: sh_lex():
- Remove incorrect check for braceexpand option before processing
  redirections of this form.

src/cmd/ksh93/COMPATIBILITY:
- Insert a brief item mentioning this.

src/cmd/ksh93/sh.1:
- Correction: these redirections do not yield a file descriptor >
  10, but > 9, a.k.a. >= 10.
- Add a brief example showing how these redirections can be used.

src/cmd/ksh93/tests/io.sh:
- Add a quick regression test.

src/cmd/ksh93/sh.1

@@ -3557,10 +3557,16 @@ forms,
 is preceded by
 .BI { varname }
 with no intervening space,
-then a file descriptor number > 10
+then a file descriptor number > 9
 will be selected by
 the shell and stored in the variable
-.IR varname .
+.IR varname ,
+so it can be read from or written to with redirections like
+.B <&
+.I $varname
+or
+.B >&
+.IR $varname .
 If
 .B >&\-
 or the any of the