MFC 259826,259997,259998:
Support soft power-off via the ACPI S5 state for bhyve guests and wire
up a virtual power button to SIGTERM:
- Implement the PM1_EVT and PM1_CTL registers required by ACPI.
- Emulate the Reset Control register at I/O port 0xcf9.
- Advertise an _S5 package.
- 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.