More fixes as a step towards ARM_MMU_EXTENDED on RPI.

- don't set CPU_CONTROL_SYST_ENABLE in arm11x6_setup for ARM_MMU_EXTENDED

- Use the new MMU defines for V6N

- pull in arm/cpuconf.h in vmparam s that VM_MAXUSER_ADDRESS gets set
  correctly.
callout_ack returns void, not bool.
disable KMS for now; it breaks UMS + (old) radeondrm.
radeon depends on firmload.

Noted by rjs@.
Use two different mutexes for tx and rx

This change splits the mutex of wm into two: one for tx and the other
for rx. By doing so, lock contentions can be reduced. We lock both for
other operations that need locking, e.g., init, stop and ioctl.

The modification doesn't change the behavior of the driver.
Make global variables of if.c MPSAFE
Add if_drain_all stub for rump