sync the code with the version in head. which the exception of

sync the code with the version in head. which the exception of
svn 275358 (M_FLOWID deprecation, only a couple of lines)
which cannot be merged.

if_lem_netmap.h, if_re_netmap.h:
- use the same (commented out) function to update the stat counters
  as in HEAD. This is a no-op here

netmap.c
- merge 274459 (support for private knote lock)
  and minor changes on nm_config and comments

netmap_freebsd.c
- merge 274459 (support for private knote lock)
- merge 274354 (initialize color if passed as argument)

netmap_generic.c
- fix a comment

netmap_kern.h
- revise the lock macros, using sx locks;
  merge 274459 (private knote lock)

netmap_monitor.c
- use full memory barriers

netmap_pipe.c
- use full memory barriers, use length from the correct queue
  (mostly cosmetic, since the queues typically have the same size)