kernel - Add critical section in lwkt_yield_quick()

* Add a critical section to protect the clearing of the LWKT reschedule
  bit against gd_tdrunq.
kernel - Fix issue with ARP packets stalling out entire network

* ARP packets can cause ARP routing table updates to occur.  An ARP
  routing table update is an expensive synchronous netmsg that is
  forwarded through *ALL* cpus.

* ARP was previously being handled by netisr 0 and on large multi-way
  machines (aka monster the 48-way opteron) under very heavy loads this
  could result in very long stalls for any packet processing forwarded
  to cpu 0.

  Stalls exceeding 200 seconds were observed on monster when a large
  number of ARP packets had to be processed.

* Implement a dedicated thread feature for the NETISR mechanism and
  modify NETISR_ARP to use it.  This takes the expensive synchronous
  ARP packet processing off the general per-cpu netisr threads.
  This thread currently runs on cpu (18 % ncpus) (NETISR_ARP == 18).

  Thus the general per-cpu (netisr 0) thread will no longer stall
  on ARP packets.

* ping latencies under extreme loads improved to (approximately):

  ping -i 0.001 monster-nr
  11735 packets transmitted, 11735 packets received, 0.0% packet loss
  round-trip min/avg/max/stddev = 0.073/0.190/27.019/0.382 ms
vkernel - Settable serial numbers for virtual disks.

  User can specify now serial numbers for their virtual disks
  from the command line.

  Example:
	./vkernel -m 128m -r root.img:VKDMYSERNO
vkernel(7) - Minor manpage adjustments.
vke(4) - Show backing tap only if one was used.
vkd(4) - Announce virtual disks upon initialization.