Re: allow benign typedef redefinitions (C11)

Here's a few more forward struct declarations turned into typedefs, for
cleanliness sake.

struct VacuumCutoffs could use a typedef, by the way ...

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"La espina, desde que nace, ya pincha" (Proverbio africano)