Kernel with 'options FAST_IPSEC' fails building.
Do we support FAST_IPSEC (described in LINT)?
I see from Matt's comments in issue 150 in 2006,
that it was likely broken at that time also.
buildkernel's initial failures are warnings, which issues build errors;
adding `nowerror' to offending files in sys/conf/files
reveals errors, like missing struct members,
e.g. if_addrlist missing in ifnet.
Chasing this a bit shows that it was an alias for if_addrhead,
which was removed in 2008, when if_addrhead was turned to if_addrheads,
as it was reimplemented (per CPU).