Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Make wait6(2), waitid(3) and ppoll(2) cancellation points. The
waitid() function is required to be cancellable by the standard. The
wait6() and ppoll() follow the other syscalls in their groups.

Reviewed by: jhb, jilles (previous versions)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week