* put #includes of opt headers and headers to get protos used by
net/netisr_dispatch.h in net/netisr.h (if !defined(_LOCORE)) (rather than
in netisr_dispatch.h itself, and potentially nowhere, respectively).
* require netisr.h to be included before netisr_dispatch.h.
* minor additional cleanup of both netisr.h and netisr_dispatch.h.
* clean up uses to remove now-unnecessary header file inclusions, and
local prototypes of the fns.
* convert netisr dispatch implementations which didn't use
netisr_dispatch.h (pc532) to use it.
