-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.

NEWS

@@ -16,6 +16,7 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   * disables the &> redirection shorthand
   * causes the 'let' arithmetic command to recognise octal numbers by
     leading zeros regardless of the setting of the 'letoctal' option
+  * causes file descriptors > 2 to be left open when invoking another program
 
 2020-08-19:
 

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

src/cmd/ksh93/sh/io.c

@@ -1094,7 +1094,7 @@ int	sh_redirect(Shell_t *shp,struct ionod *iop, int flag)
 	Namval_t *np=0;
 	int isstring = shp->subshell?(sfset(sfstdout,0,0)&SF_STRING):0;
 
-	if(flag==2)
+	if(flag==2 && !sh_isoption(SH_POSIX))
 		clexec = 1;
 	if(iop)
 		traceon = sh_trace(shp,NIL(char**),0);

src/cmd/ksh93/tests/io.sh

@@ -616,5 +616,13 @@ then	set -o posix
 	[[ $(< aha2.txt) == ok2 ]] || err_exit '&> does not redirect stderr'
 fi
 
+# In POSIX mode, file descriptors > 2 should remain open when invoking another proram
+if	[[ -o ?posix ]]
+then	(set -o posix; exec 7>ok.txt; "$SHELL" -c 'print ok >&7' 2>/dev/null)
+	[[ $(<ok.txt) == ok ]] || err_exit 'File descriptors > 2 not inherited in POSIX mode'
+fi
+(exec 7>bad.txt; "$SHELL" -c 'print bad >&7' 2>/dev/null)
+[[ $(<bad.txt) == '' ]] || err_exit 'File descriptors > 2 inherited without POSIX mode' "(got $actual)"
+
 # ======
 exit $((Errors<125?Errors:125))