Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

make 'make universe' not complain about
In file included from /src/sys/modules/powermac_nvram/../../dev/powermac_nvram/powermac_nvram.c:38:
@/dev/ofw/ofw_bus.h:36:24: error: ofw_bus_if.h: No such file or directory
I am not sure for how long this had not worked and if it was just the
latest vimage commit that had revealed this or if nobody had built
universe successfully in a while. Btw, the tinderbox did not complain
either so that is probably the reason noone had noticed.

Add missing headers.

Add device to access and modify Open Firmware NVRAM settings in
PowerPC-based Apple's machines and small utility to do it from
userland modelled after the similar utility in Darwin/OSX.

Only tested on 1.25GHz G4 Mac Mini.

MFC after: 1 month