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

Convert sbin/mdconfig/tests from prove format tests to ATF format tests

As a side effect...
1. The tests now checks for the root user before continuing with kyua, which is
more visible than the test being skipped with the TAP protocol
2. The tests work with devices that aren't /dev/md0 by caching the device
attached during the test to a file, and later use the cached information to
detach the device in the cleanup routine
3. The tests no longer require perl to run

MFC after: 1 week
PR: 191191
Sponsored by: EMC / Isilon Storage Division