Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.Approved by: re (implicit)Sponsored by: The FreeBSD Foundation
Move the definition of the struct unrhdr into a separate header file,to allow embedding the struct. Add init_unrhdr(9) initializer, whichsets up preallocated unrhdr.Reviewed by: alcTested by: pho, bf