Fix truncating of files with <>;file combined with <#pattern

The issue with truncating files was caused by out-of-sync streams.
Details and discussion: https://github.com/att/ast/issues/61

src/cmd/ksh93/sh/io.c: sh_iorestore():
- To be safe, sync all streams before restoring file descriptors.

src/cmd/ksh93/tests/io.sh:
- Add two regression tests for truncating files with this
  combination of redirections.
- The second test, which invokes a -c script, is disabled for now
  as this triggers another corner case bug involving the SH_NOFORK
  optimisaton for -c scripts. That fix is for another commit.

(cherry picked from commit 18fb64840365c2ff4608188e5487bd79d08f67d1)

src/cmd/ksh93/sh/io.c

@@ -1749,6 +1749,11 @@ void	sh_iorestore(Shell_t *shp, int last, int jmpval)
 	register int 	origfd, savefd, fd;
 	int flag = (last&IOSUBSHELL);
 	last &= ~IOSUBSHELL;
+	/*
+	 * There was an issue with truncating files (see 'ftruncate' below) that was caused by
+	 * out-of-sync streams. So, to be safe, sync all streams before restoring file descriptors.
+	 */
+	sfsync(NULL);
 	for (fd = shp->topfd - 1; fd >= last; fd--)
 	{
 		if(!flag && filemap[fd].subshell)