MFC r256502:
Similar to debug.iosize_max_clamp sysctl, introduce
devfs_iosize_max_clamp sysctl, which allows/disables SSIZE_MAX-sized
i/o requests on the devfs files.
MFC r256504:
Add a sysctl kern.disallow_high_osrel which disables executing the
images compiled on the world with higher major version number than the
high version number of the booted kernel.  Default to disable.