[Swan-commit] Changes to ref refs/heads/master

[D. Hugh Redelmeier]

New commits:
commit 27fb14f528e76f3a0e08f8cbf42ff05427901ccd
Author: D. Hugh Redelmeier <hugh at mimosa.com>
Date:   Thu Feb 21 11:22:40 2019 -0500

    pluto: fiddle with list_entry
    
    - add and correct comments to make the invariants clearer
    
    - add detached_list_entry() and make remove_list_entry()
      require that the entry actually be on the list.
    
    - add a bunch of invariant-checking passerts.
    
    Prompted by a clang warning.