Don't unconditionally reset the hardware debug registers in cpu_exit(), reset them only if they were previously in use.  Unconditionally resetting the registers wipes them out frequently, which interferes with their use for kernel debugging.
Don't unconditionally reset the hardware debug registers in cpu_exit(),
reset them only if they were previously in use.  Unconditionally
resetting the registers wipes them out frequently, which interferes
with their use for kernel debugging.

While I'm here, be less verbose in the associated comment of a
neighboring function.

Noticed by:	bde

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase