Grow: add 'forked' option to reshape_container.

This is a better match for reshape_array() and means that
"mdadm --grow --continue" will run in the foreground, which
makes more sense.

Signed-off-by: NeilBrown <neilb@suse.de>
Grow: split continue_via_systemd into a separate function.

This allows it to be used for containers too.

Signed-off-by: NeilBrown <neilb@suse.de>
Reshape: use systemd to continue containers as well as native arrays.

Signed-off-by: NeilBrown <neilb@suse.de>