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

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

Revise markup.

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