Add locking and mark MPSAFE.

• Add a mutex to protect the softc.
• Use callout(9) instead of timeout(9).
• Consolidate duplicated detach routines into a bus-independent detach routine.
• Add an extra sleep lock flag (MSESC_READING) to prevent other readers from reading while the first reader is copying data out of sc_bytes[] via uiomove().
• Use bus_*() instead of bus_space_*().

Tested by: nyan