Bulk sync of pf changes from head, in attempt to fixup broken build I
made in r263029.

Merge r257186,257215,257349,259736,261797.

These changesets split pfvar.h into several smaller headers and make
userland utilities to include only some of them.
Merge r256514:
  - While we are spreading the counter(9) across network stack, more userland
  tools would need to know about the counter_u64_t type. Allow to include
  sys/counter.h from userspace.