This should not have any effect on behaviour. The goal is to align
with the pattern that I think we should follow:

Object initialization:
 - put() is the place to create references from other objects to the
   newly created object. In this case, adding the transport to
   discovery->transports was moved from new() to put, and adding the
   transport to device->transports was moved from set_state() to
   put().

Object destruction:
 - unlink() undoes put() and removes all references from other objects
   to the object being unlinked. In this case setting the
   device->transports pointer to NULL was moved from set_state() to
   unlink(), and setting the discovery->transports pointer to NULL was
   moved from free() to unlink().
 - free() undoes new(), but also calls unlink() so that object owners
   don't need to remember to call unlink() before free().