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

Add locking to scd(4) and mark MPSAFE.
- Actually use existing per-softc mutex.
- Use mutex in cdev routines and remove D_NEEDGIANT.
- Use callout(9) instead of timeout(9).
- Don't check for impossible conditions (e.g. SCDINIT being clear).
- Use bus_*() instead of bus_space_*().

Tested by: no one

Do the dreaded s/dev_t/struct cdev */
Bump __FreeBSD_version accordingly.

Fix for GEOM.

Tested on: i386

- Convert to newbus, bus_space etc.
- Move to MI space.

Tested on: i386