Rename the confusing neutral annotation in CClosure.
Rename the confusing neutral annotation in CClosure.

The Norm name was confusing enough to have introduced a few kernel
bugs, so it deserved to be changed as suggested in #13274.
Contrarily to what it seemingly meant, it was actually standing
for neutral terms rather than normal ones. I have kept the 4-letter
naming scheme for simplicity and renamed it into Ntrl.

gares/coq,coq/coq,SkySkimmer/coq,gares/coq,SkySkimmer/coq,gares/coq,ejgallego/coq,coq/coq,JasonGross/coq,coq/coq,silene/coq,coq/coq,silene/coq,gares/coq,JasonGross/coq,Zimmi48/coq,Zimmi48/coq,JasonGross/coq,SkySkimmer/coq,JasonGross/coq,Zimmi48/coq,ejgallego/coq,ejgallego/coq,ejgallego/coq,silene/coq,gares/coq,Zimmi48/coq,SkySkimmer/coq