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

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

Improve consistency of include file guards in src/sys/sys by terminating
them with '_', as well as beginning with '_'.

Observed by: bde

Remove __P

o Include file to provide prototypes for regression testing-specific
system calls/et al. Not much in there just yet, but will most likely
grow.

Obtained from: TrustedBSD Project