In ipfilter(4) there is the ipftest(1) program, that compiles half of the
ipfilter code as userland application. To reduce kernel structure knowledge
include if_var.h only if a file is compiled with _KERNEL defined.
In !_KERNEL case, provide our own definition of struct ifnet, that will
satisfy ipftest(1). This was already done earlier to struct ifaddr in
r279029. Protect the definition with _NET_IF_VAR_H_, since kernel part
of ipfilter may include if_var.h and ip_compat.h.

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