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