Manage: fix checks for removal from a container.

Manage: fix checks for removal from a container.

We must only remove from a container if the device isn't a
member of any member array.
To check we look at the 'holders' directory in sysfs.

We currently skip that check if ->devname is "detached", however
that can never be true since the change that introduced
add_detached().

Also sysfs_unique_holder returns status in 'errno' which isn't
entirely safe as e.g. closedir() is probably allowed to clear it.

So make sysfs_unique_holder return an unambigious value, and us
it to decide what to report.

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