Add cio.Load for loading io set

This adds a `Load` Opt for cio to load a tasks io/fifos without
attaching or starting the copy routines.

It adds the load method in `ctr` by default so that fifos or other IO
are removed from disk on delete methods inbetween command runs.  It is
not the default for all task loads for backwards compat. and a user may
want to keep io around to reuse or if log files are used.

Fixes #2421

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cio/io.go

@@ -255,3 +255,14 @@ func (l *logIO) Wait() {
 func (l *logIO) Close() error {
 	return nil
 }
+
+// Load the io for a container but do not attach
+//
+// Allows io to be loaded on the task for deletion without
+// starting copy routines
+func Load(set *FIFOSet) (IO, error) {
+	return &cio{
+		config:  set.Config,
+		closers: []io.Closer{set},
+	}, nil
+}