bhyve: Extend mevent to support updating timers

This will be used by a new PL031 implementation to provide an RTC for
arm64 guests.

Reviewed by: jhb
MFC after: 2 weeks
Obtained from: CheriBSD

Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

bhyve: clean up trailing whitespaces

Clean up trailing whitespaces. No functional changes.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D33681

bhyve: Add support for EVFILT_VNODE mevents.

This allows registering an event to watch for changes to a file's
attributes. This is a bit imperfect as it would be nice to have a way
to determine if an fd can use EVFILT_VNODE successfully. mevent's
current structure does not permit that and a failure to register a
single kevent impacts several other kevents.

Reviewed by: grehan, markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D30503

bhyve: add backend rx backpressure to virtio-net

If a VM is flooded with more ingress packets than the guest OS
can handle, the current virtio-net code will keep reading those
packets and drop most of them as no space is available in the
receive queue. This is an undesirable receive livelock, which
is a waste of CPU and memory resources and potentially opens to
DoS attacks.
With this change, virtio-net uses the new netbe_rx_disable()
function to disable ingress operation in the backend while the
guest is short on RX buffers. Once the guest makes more buffers
available to the RX virtqueue, ingress operation is enabled again
by calling netbe_rx_enable().

Reviewed by: bryanv, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20987

various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

Extend the ACPI power management support to wire a virtual power button up
to SIGTERM when ACPI is enabled. Sending SIGTERM to the hypervisor when an
ACPI-aware OS is running will now trigger a soft-off allowing for a graceful
shutdown of the guest.
- Move constants for ACPI-related registers to acpi.h.
- Implement an SMI_CMD register with commands to enable and disable ACPI.
Currently the only change when ACPI is enabled is to enable the virtual
power button via SIGTERM.
- Implement a fixed-feature power button when ACPI is enabled by asserting
PWRBTN_STS in PM1_EVT when SIGTERM is received.
- Add support for EVFILT_SIGNAL events to mevent.
- Implement support for the ACPI system command interrupt (SCI) and assert
it when needed based on the values in PM1_EVT. Mark the SCI as active-low
and level triggered in the MADT and MP Table.
- Mark PCI interrupts in the MP Table as active-low in addition to level
triggered.

Reviewed by: neel

Add simplistic periodic timer support to mevent using kqueue's
timer support. This should be enough for the emulation of
h/w periodic timers (and no more) e.g. some of the 8254's
more esoteric modes that happen to be used by non-FreeBSD o/s's.

Approved by: re@ (blanket)

Import of bhyve hypervisor and utilities, part 1.
vmm.ko - kernel module for VT-x, VT-d and hypervisor control
bhyve - user-space sequencer and i/o emulation
vmmctl - dump of hypervisor register state
libvmm - front-end to vmm.ko chardev interface

bhyve was designed and implemented by Neel Natu.

Thanks to the following folk from NetApp who helped to make this available:
Joe CaraDonna
Peter Snyder
Jeff Heller
Sandeep Mann
Steve Miller
Brian Pawlowski