Submitted by: CAIA (http://caia.swin.edu.au)
Minor cleanup of the includes used by the various source files, including annotations of why certain headers are used.
Usual moving of code with no changes from ipfw2.c to the newly created files, and addition of prototypes to ipfw2.h I have added forward declarations for ipfw_insn_* in ipfw2.h to avoid a global dependency on ip_fw.h