-o posix: allow invoked programs to inherit FDs > 2

If there are file descriptors > 2 opened with 'exec' or 'redirect',
ksh93 has always closed them when invoking another pogram. This is
contrary to POSIX which states:
    Utilities other than the special built-ins […] shall be invoked
    in a separate environment that consists of the following. The
    initial value of these objects shall be the same as that for
    the parent shell, except as noted below.
    * Open files inherited on invocation of the shell, open files
      controlled by the exec special built-in plus any
      modifications, and additions specified by any redirections to
      the utility
    * […]
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_12

src/cmd/ksh93/sh/io.c: sh_redirect():
- When flag==2, do not close FDs > 2 if POSIX mode is active.

src/cmd/ksh93/tests/io.sh:
- Regress-test inheriting FD 7 with and without POSIX mode.

src/cmd/ksh93/sh.1:
- Update.

src/cmd/ksh93/sh.1

@@ -6817,7 +6817,8 @@ command.
 Any file descriptor numbers greater than
 .B 2
 that are opened with this mechanism are closed when invoking another program,
-unless explicitly redirected to themselves as part of that invocation.
+unless they are explicitly redirected to themselves as part of that invocation
+(e.g. \fb4>&4\fR) or the \fBposix\fR shell option is active.
 .TP
 \(dg \f3return\fP \*(OK \f2n\^\fP \*(CK
 Causes a shell
@@ -7038,6 +7039,7 @@ to fail or zero if no command has failed.
 .B posix
 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,
 enables octal numbers in \fBlet\fR shell arithmetic (see \fBletoctal\fR), and
 disables the \fB&>\fR redirection shorthand.
 .TP 8