Fix v=$(<file) for closed FD 0,1,2 (rhbz#1066589)

var=$(< file) now reads the file even if the standard inout,
standard output and/or standard error file descriptors are closed.

Original patch:
https://src.fedoraproject.org/rpms/ksh/blob/642af4d6/f/ksh-20120801-filecomsubst.patch

src/cmd/ksh93/sh/io.c: sh_redirect():
- When processing the '<' redirector as part of $(< ...), i.e. if
  flag==3, make sure the FD of the file to read is > 2 by calling
  sh_iomovefd(). Unlike the RedHat patch, this checks for flag==3
  to avoid unnecessary sh_iomovefd() calls for normal redirections,
  as there was no bug with those.

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

NEWS

@@ -9,6 +9,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   to lose the output of the commands they ran. This bug occurred when ksh was
   compiled with the SHOPT_SPAWN compile-time option.
 
+- Bugfix: var=$(< file) now reads the file even if the standard inout, standard
+  output and/or standard error file descriptors are closed.
+
 2020-09-20:
 
 - Bugfix: when whence -v/-a found an "undefined" (i.e. autoloadable) function

src/cmd/ksh93/sh/io.c

@@ -1266,6 +1266,8 @@ int	sh_redirect(Shell_t *shp,struct ionod *iop, int flag)
 				if(flag==SH_SHOWME)
 					goto traceit;
 				fd=sh_chkopen(fname);
+				if(flag==3)			/* make sure that $(<file) works...	*/
+					fd=sh_iomovefd(fd);	/* ...with stdin/stdout/stderr closed	*/
 			}
 			else if(sh_isoption(SH_RESTRICTED))
 				errormsg(SH_DICT,ERROR_exit(1),e_restricted,fname);

src/cmd/ksh93/tests/io.sh

@@ -462,6 +462,7 @@ got=$(<$tmp/22.out)
 
 tmp=$tmp $SHELL 2> /dev/null -c 'exec 3<&1 ; exec 1<&- ; exec > $tmp/outfile;print foobar' || err_exit 'exec 1<&- causes failure'
 [[ $(<$tmp/outfile) == foobar ]] || err_exit 'outfile does not contain foobar'
+[[ $(<$tmp/outfile) == foobar ]] <&- >&- 2>&- || err_exit '$(<file) does not work with stdin, stdout and/or stderr closed'
 
 print hello there world > $tmp/foobar
 sed  -e 's/there //' $tmp/foobar  >; $tmp/foobar