This is the struct type used as set/getsockopt() option data with
SO.ACCEPTFILTER, which is also only declared on this same limited
set of BSD-ish targets.

In theory this could be aliased over to std.posix as well, but I
think for a corner case like this, it's not unreasonable for a
user that is avoiding uneccessary std.c references to access it as
"posix.system.accept_filter_arg" (which would still work fine if,
in the future, FreeBSD escapes its libc dep and defines it in
std.os.freebsd).