Most of it doesn't need to be public; pull it into epan/ipv4.c. Pull
the two routines that *are* used outside epan/ftypes/ftype-ipv4.c into
epan/ipv4.h as static inline functions.
This allows some optimization, and makes epan/ipv4.h more like epan/ipv6.h.
Change-Id: I80229acde559d810aecec2acd5c995076440c181
Reviewed-on: https://code.wireshark.org/review/24071
Reviewed-by: Guy Harris <guy@alum.mit.edu>