kernel - Revamp ifsq_watchdog*() a bit

kernel - Revamp ifsq_watchdog*() a bit

* Add a flags argument to ifsq_watchdog_init() and implement two
  flags:

  IF_WDOG_ALLTICKS	Watchdog callback occurs each second
			prior to terminal count, and on terminal
			count.

  IF_WDOG_LASTTICK	Watchdog callback occurs one second prior
			to terminal count, and on terminal count.

  0			Watchdog callback only occurs on terminal
			count.

  Note that the ifnet lock is fully serialized when a watchdog
  callback is made, so generally speaking to avoid hicups you
  only want to use IF_WDOG_LASTTICK.

* This feature allows the watchdog code to flush any tx buffers
  (call *_txeof()) that have completed transmission.  Many NICs
  and virtual NICs don't bother to generate interrupts to clean-up
  such buffers when the rings are only lightly loaded.

* Fix spurious vtnet watchdog events on lightly loaded vtnet
  interfaces, loosly using the same code that FreeBSD uses
  (just txeof from the second-prior watchdog callback and then
  check on terminal count).