poll - Fix events == 0 handling for TAP and TUN, fix console spam

* BSD poll() supports events == 0 representing a "HUP ONLY" event.
Most of our devices properly support this but TAP and TUN did n

poll - Fix events == 0 handling for TAP and TUN, fix console spam

* BSD poll() supports events == 0 representing a "HUP ONLY" event.
Most of our devices properly support this but TAP and TUN did not.

Fix TAP and TUN to support this feature. This fixes openvpn()
when operating under heavier loads.

* Fix console spam on non-debug kprintf()s by rate-limiting any
such spam if it occurs. If a device doe snot support poll()'s
HUP ONLY feature, the loop deregistration path will be hit and
generate a kprintf(). We don't want console spam to stall the
cpu.

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kernel: No need to handle mbuf allocation failures if use M_WAITOK

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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kernel: Rename BUILDING_WITH_KERNEL to KERNBUILDDIR.

Fits better with e.g. KERNCONFDIR.

Add a small paragraph in kmod.mk which was taken from FreeBSD.