Make the distribution of in_ and in6_ functions in in_pcb.c and

Make the distribution of in_ and in6_ functions in in_pcb.c and
in6_pcb.c consistent, to ease comparing the code.  Move all inet6
functions to in6_.  Bring functions in both source files in same
order.  Cleanup the include section.  Now in_pcb.c is a superset
of in6_pcb.c.  The latter contains all the special implementations.
Just moving arround, no code change intended.
OK mpi@