pkg/mount: add more sharesubtree options

Signed-off-by: Vincent Batts <vbatts@redhat.com>
pkg/mount: testing for linux sharedsubtree mounts

* shared
* shared/slave
* unbindable
* private

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #8897 from vbatts/vbatts-mount_sharedsubtree

pkg/mount: sharedsubtree options and testing