Revert <> redir FD except in posix mode (re: eeee77ed, 60516872)

eeee77ed implemented a POSIX compliance fix that caused a potential
incompatibility with existing ksh scripts; it made the (rarely
used) read/write redirection operator, <>, default to file
descriptor 0 (standard input) as POSIX specified, instead of 1
(standard output) which is traditional ksh93 behaviour. So ksh
scripts needed to change all <> to 1<> to override the new default.

This commit reverts that change, except in the new posix mode.

src/cmd/ksh93/sh/lex.c:
- Make FD for <> default to 0 in POSIX mode, 1 otherwise.

src/cmd/ksh93/tests/io.sh:
- Revert <> regression test changes from 60516872; we no longer
  need 1<> instead of <> in ksh code.

src/cmd/ksh93/sh.1

@@ -3423,7 +3423,8 @@ otherwise, the file is created.
 Open file
 .I word\^
 for reading and writing
-as standard input.
+as standard output.
+If the \fBposix\fR option is active, it defaults to standard input instead.
 .TP
 .BI <>; word
 The same as
@@ -7040,6 +7041,7 @@ to fail or zero if no command has failed.
 Enable POSIX standard compatibility mode. This option
 is on by default if ksh is invoked as \fBsh\fR. It
 causes file descriptors > 2 to be left open when invoking another program,
+makes the \fB<>\fR redirection operator default to standard input,
 enables octal numbers in \fBlet\fR shell arithmetic (see \fBletoctal\fR), and
 disables the \fB&>\fR redirection shorthand.
 .TP 8