Start splitting pfvar.h into internal and external parts.

- Provide pf_altq.h that has only stuff needed for ALTQ.
- Start pf.h, that would have all constant values and
  eventually non-kernel structures.
- Build ALTQ w/o pfvar.h, include if_var.h, that before
  came via pollution.
- Build tcpdump w/o pfvar.h.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Fix an itt instruction. We need to execute both the mov and b instructions
when building for Thumb.