When root is created with su_root_create() or cloned with su_clone_start(),
the resulting root is obtained by the calling or created thread,
respectively.

The root can be released with su_root_release() and another thread can
obtain it.

The function su_root_has_thread() can be used to check if a thread has
obtained or released the root.

Implementation upgraded the su_port_own_thread() method as su_port_thread().

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