runtime: remove untyped allocation of ParFor

Now it's two allocations. I don't see much downside to that,
since the two pieces were in different cache lines anyway.

Rename 'conservative' to 'cgo_conservative_type' and make
clear that _cgo_allocate is the only allowed user.

This depends on CL 141490043, which removes the other
use of conservative (in defer).

LGTM=dvyukov, iant
R=khr, dvyukov, iant
CC=golang-codereviews, rlh
https://golang.org/cl/139610043

src/cmd/api/goapi.go

@@ -404,6 +404,7 @@ func (w *Walker) parseFile(dir, file string) (*ast.File, error) {
 			" mutex struct{};" +
 			" note struct{};" +
 			" p struct{};" +
+			" parfor struct{};" +
 			" slicetype struct{};" +
 			" stkframe struct{};" +
 			" sudog struct{};" +