Introduce support for Mandatory Access Control and extensible
kernel access control.

Replace 'void *' with 'struct mac *' now that mac.h is in the base
tree. The current POSIX.1e-derived userland MAC interface is
schedule for replacement, but will act as a functional placeholder
until the replacement is done. These system calls allow userland
processes to get and set labels on both the current process, as well
as file system objects and file descriptor backed objects.