Insert a series of place-holder function pointers in mac_policy.h for
entry points that will be inserted over the life-time of the 6.x branch,
including for:

• New struct file labeling (void * already added to struct file), events, access control checks.
• Additional struct mount access control checks, internalization/ externalization.
• mac_check_cap()
• System call enter/exit check and event.
• Socket and vnode ioctl entry points.

MFC after: 3 days