Add following three network protocol threads running mode:

Add following three network protocol threads running mode:
1) BGL (default)
2) Adaptive BGL.  Protocol threads run without BGL by default.  BGL will be
   held if the received msg does not have MSGF_MPSAFE turned on the ms_flags
   field
3) No BGL (experimental)

The code on the main path is done by dillon@

Following three sysctls and tunables are added to adjust the "mode":
net.netisr.mpsafe_thread
net.inet.tcp.mpsafe_thread
net.inet.udp.mpsafe_thread

They have same set of values,
0 (default) -- BGL
1 	    -- Adaptive BGL
2 	    -- No BGL

NETISR_FLAG_MPSAFE is added (netisr.ni_flags), so that:
- netisr_queue() and schednetisr() could set MSGF_MPSAFE during msg
  initialization
- netisr_run() (called by ether_input_oncpu()) could hold BGL based on this flag
  before calling netisr's handler

PR_MPSAFE is added (protosw.pr_flags), so that tranport_processing_oncpu() could
hold BGL before calling protocol's input handler

Kernel API changes:
- The thread parameter to netmsg_service_loop() must be supplied (running mode)
  and it must have the type of "int *"
- netisr_register() takes additional flags parameter to indicate whether its
  handler is MPSAFE (NETISR_FLAG_MPSAFE) or not

Reviewed-by: dillon@