As far as I can tell, the sole obstacle to configuring, compiling, and
linking both FAST_IPSEC (src/sys/netipsec) and `options INET6' is
three or four references to the FreeBSD version of a KAME header file,
sys/netinet6/ip6_ecn.h. That file does not exist in NetBSD-current;
intsead the functions ip6_ecn_ingress() and ip6_ecn_egress() are
defined in sys/netinet/ip_ecn.h, inside #ifdef INET6/#endif.

Rather than dink a half-dozen more "#ifdef __FreeBSD__ " into the
fast-ipsec source, I'd prefer to move those two functions into
netinet6/ip6_ecn.h, and preserve the existing namespace by having
netinet/ip_ecn.h do

#ifdef INET6
#include <netinet6/ip6_ecn.h>
#endif

Comments? Or is it likely FreeBSD would buy back the newer(?)
NetBSD-style, unified header?

If at all possible, I'd like to resolve this today-ish...