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

Follow-up r297842: Rework header generation to fix always rebuilding.

This reworks the handling of common headers to just include the needed
logic rather than invoke MAKE. This avoids the problem listed in r297842
and avoids other dependency tracking issues.

Pointyhat to: bdrewery
Reported by: Nikolai Lifanov <lifanov@mail.lifanov.com>
Sponsored by: EMC / Isilon Storage Division