kernel - Add per-process capability-based restrictions

* This new system allows userland to set capability restrictions which
turns off numerous kernel features and root accesses. These restricti

kernel - Add per-process capability-based restrictions

* This new system allows userland to set capability restrictions which
turns off numerous kernel features and root accesses. These restrictions
are inherited by sub-processes recursively. Once set, restrictions cannot
be removed.

Basic restrictions that mimic an unadorned jail can be enabled without
creating a jail, but generally speaking real security also requires
creating a chrooted filesystem topology, and a jail is still needed
to really segregate processes from each other. If you do so, however,
you can (for example) disable mount/umount and most global root-only
features.

* Add new system calls and a manual page for syscap_get(2) and syscap_set(2)

* Add sys/caps.h

* Add the "setcaps" userland utility and manual page.

* Remove priv.9 and the priv_check infrastructure, replacing it with
a newly designed caps infrastructure.

* The intention is to add path restriction lists and similar features to
improve jailess security in the near future, and to optimize the
priv_check code.

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